Who will replace Boris Johnson as the next Mayor of London?

Boris Johnson announced yesterday that he will leave City Hall in May 2016. This means the official fight to become the next Mayor of London can finally begin.

A number of campaigns are already well underway behind the scenes. Let's take a look at them. Here are the Labour ones first.

Christian Wolmar

Transport expert and historian Christian Wolmar is still the only officially declared candidate so far. For such a low-profile public figure, he already has an impressive campaign machine behind him and has quietly built up a significant level of support from influential figures in the Labour movement including Neil Lawson from Compass.

However, this doesn't translate into votes and one recent poll found that he has the least support of all the mooted Labour candidates so far. However, unlike his colleagues, Wolmar is well into the process of developing some actual policies for London. Hopefully his opposition to the mayor's road-building policies and his wider transport agenda will play a big part in the rest of the campaign.

Tessa Jowell

Jowell yesterday released a statement suggesting that she will run in 2016. She said: "There will be much speculation about candidates… I will be taking this time to prepare my potential offer to Londoners," and added "but this is not the time for formal decision or declaration."

Jowell is the current frontrunner on the Labour side, but she is by no means a sure thing. Her closeness to Tony Blair and support for policies such as the Iraq war could count against her among the left-leaning London Labour party.

On the other hand, she is the only current candidate with a record of actually managing a multi-billion pound budget, because of her work on the Olympics. She also has the kind of crossover appeal that most of her rivals do not. She is one of the few people to have worked with both Ken Livingstone and Boris Johnson and still maintains good relations with both of them. Many Tories like and respect her and Boris has previously told friends that she would be a "formidable" opponent. Whether or not this counts in her favour is another matter.

Diane Abbott

Abbott is the current second favourite according to the polls. Her independence and left-leaning politics make her a better fit than Jowell as far as the London Labour party are concerned and she also has a higher public profile than most of her rivals. Like Livingstone in 2000, she is also independent from the Labour leadership. Doubts remain about whether she would be a 'safe pair of hands' at City Hall however. She will need to convince the party that she has a clear agenda for London and not just the ambition to be mayor.

Sadiq Khan

Khan has a number of advantages. As shadow minister for London and head of campaigning in the capital, he is widely credited with Labour's excellent showing in this year's local elections in the capital. He is also the favoured candidate of the Labour leadership. However, this is both a blessing and a curse. On the one hand he will receive all of the support Miliband and those around him can supply. On the other hand, receiving such support may well count against him. In 2000, Tony Blair was at the peak of his powers and put all of his formidable resources behind the election of Frank Dobson. Dobson came a poor third behind Livingstone and Steve Norris. History has shown that Londoners like their mayor to be independent from individual parties. Khan is very much a central figure in the Labour party machine.

David Lammy

Lammy came close to standing against Ken Livingstone at the last Labour London mayoral selection in 2010. In the end he decided against it and instead threw his weight behind Livingstone's campaign. This time he's almost certain to stand. He has a number of key advantages, not least his performance during the 2011 riots when he was widely respected for standing up for his community, in contrast to the mayor's initial refusal to return home from his holidays. He is also independent from the leadership and has been highly critical of Miliband at times. However, London Labour figures have suggested to Politics.co.uk that Lammy lacks the breadth of support he would need to win the bid and has made a number of enemies both within and outside the party. One very well known right-wing broadcaster recently told me that they would immediately launch their own campaign to be mayor, if Labour picked Lammy. Without wanting to reveal who this person is, it is a frightening prospect.

Best of the rest

Other Labour candidates being talked about include Lord Adonis, Margaret Hodge, Stella Creasy, and Robin Wales. Of these Stella Creasy is the most interesting choice. Her campaigning work on payday loans and her polished communication skills could help her win the nomination. However, she is fairly unknown among the wider electorate and would probably struggle if the Tories choose a reasonably well-known figure.

One interesting thing about all of the leading Labour candidates is that none of them are obvious breaks from the Ken Livingstone era. I recently asked Ken what he thought of the current selection. The normally outspoken former mayor told me that he would be perfectly happy with any of them becoming mayor. Make of that what you will.

Now let's take a look at the potential Conservative candidates.

A number of big names have already been mooted for the Conservatives including Karren Brady and Sebastian Coe. However these all appear to have ruled themselves out. So who else is there?

Zac Goldsmith

Personally I think the Tories would be crazy not to consider Goldsmith. He is a charismatic and forceful personality with obvious cross-party appeal. His record in SW London and his environmental campaigning means that he would likely pick up second preferences both from the Lib Dems and the Greens. His independence from and strong criticism of the Conservative leadership also means that he would be hard for Labour to attack as a typical Tory. Although his immense personal wealth and background may count against him, it would also mean that he would be in a position to fund his own campaign free from the usual influence of property developers and trade unions. Goldsmith seems to me to be an obvious choice. Whether or not he wants the job is another matter.

James Cleverly

Cleverly is a relatively little-known London Assembly member and chairman of the London fire authority. However, like Goldsmith, he does have potential for cross-party appeal and has demonstrated an ability to work with opponents during his time with the London fire brigade. All the signs are that he is seriously considering a run for mayor. However, lack of name recognition could be his biggest barrier.

Best of the rest

The rest of the field looks fairly sparse. Current deputy mayor and former runner up for the Tory candidacy Victoria Borwick has privately ruled out another bid. Previous favourite Kit Malthouse also appears to be out of the running now after being selected for a safe seat at Westminster. Their colleague on the London Assembly Andrew Boff is another possibility. Like Goldsmith he is certainly not a typical Conservative and could attract some cross-pary support. However, like Cleverly he is also fairly unknown.

There are probably a number of other Tory big hitters who have held back from appearing interested in the mayoralty while it was still possible that Boris might serve a third term. Now that possibility has been ruled out, we should start to see a few more interesting names coming forward.

So what about the Liberal Democrats? So far there only appears to be one obvious candidate in the running.

Caroline Pidgeon

London Assembly member Caroline Pidgeon is a formidable campaigner and was widely expected to stand at the last mayoral election. With both Ken, Boris and Brian Paddick out of the picture, she is far more likely to do so this time. Her chances of actually winning the mayoralty are clearly very slim, however the Lib Dems are more concerned with maintaining their current level of representation on the London Assembly, whose elections run at the same time as the mayoralty. As a current member, Pidgeon would be in a good position to raise their profile.

The Greens

Current London Assembly Members Jenny Jones and Darren Johnson are both expected to stand down at the next elections and are thought to be keen to avoid standing for the mayoralty again. Other possible names include former candidate and current Green councillor, Sian Berry. Berry did well as the party's candidate in the 2008 mayoral elections. However, the main name in the running at the moment appears to be the current leader of the party Natalie Bennett. Like the Lib Dems, the Greens know that whoever they choose, they will be unlikely to win outright. However, the mayoral election is a huge national platform which the party could use to raise the profile of Bennett and the wider party.

This is the first mayoral election without either Johnson or Livingstone on the ticket and it is also the first mayoral election without any obvious winners. So far the next mayoral election looks like it will be a much lower key affair than the four we have had so far, but this could quickly change. After all, almost nobody saw Boris Johnson coming two years before the 2008 mayoral election. A similar big surprise could be about to emerge in the run up to 2016 as well.