If a Bill is amended in the Lords, or the Lords reject or change amendments made by MPs, it must return to the Commons for MPs to endorse or reject the opinion of the Lords. Proceedings on the Consideration of Lords Amendments are programmed as with all other stages. The Government tables motions agreeing or disagreeing with Lords Amendments. It may also move amendments in lieu of changes rejected.
Unless the Commons acquiesce with the alterations made by peers, the Bill will be passed back to the Lords for their consideration. The Bill is communicated with 'Reasons', which outline why MPs do not agree with the Upper House. Technically, these Reasons are drawn up by a Reasons Committee, which is sent out after the end of Consideration of Lords Amendments. It reports the Reasons and the Bill is passed back.
If the Lords still cannot agree to the provision, they re-amend it and send it back (they communicate with the Commons via 'Messages'). The Commons consider the changes and may again reject them or make further changes in lieu. The Bill goes back to the Lords.
In theory, this so-called 'ping-pong' can continue indefinitely until consensus is reached but this is limited by the availability of Parliamentary time, the length of the Session and the likelihood that one or other side will put forward an acceptable compromise. In most cases, if a Bill does not receive Royal Assent before prorogation, it is lost.
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