How parliament plans to pull back from the Brexit cliff-edge

But lawmakers have already voted to overturn that convention and the group behind the latest plan has so far used control of the agenda on two other occasions to hold votes on alternative Brexit options.

The group is due to have control of some parliamentary time on Wednesday, and the lawmaker leading the legislation told BBC Radio they would use that to carry out the process that needs to be completed before the bill is debated. The plan is then to debate the bill on Thursday.

New laws in Britain require the approval of both houses of parliament - the elected House of Commons, the main legislative body, and the unelected House of Lords, which is principally seen as a revising and refining mechanism, but does technically have the power to block laws.

The Commons requires legislation to pass through several stages of voting and revisions. This process can take months, but can also be compressed. Laws have previously passed through in a day when necessary.

The Lords has a similar multi-stage process, but unlike the Commons, there is no ability to time-limit or control the debate. Legislation usually passes through the Lords more slowly, but there is precedent for rapid approval.

Once approved by both Houses, the legislation requires Royal Assent - normally a formality - before it becomes law.

The proposed legislation was published on Tuesday. It would require May to propose an extension to the Brexit negotiating period, the details of which would need to be approved by parliament.

It does not set out how long the government should seek to delay Brexit for, and notes that both parliament and the EU could propose a different length of extension.

Any extension requires the unanimous approval of the EU.

The plan relies on there being sufficient will among lawmakers in both chambers to pass the legislation before April 10, when May is due to attend a EU summit to discuss the next steps on Brexit.

Leaving without a deal on April 12 is the default legal option if Britain cannot present another viable option to EU leaders at that summit.

The government hopes that if the deal already agreed between May and the EU, which has already been rejected by lawmakers three times, is approved this week, it can secure a short extension without having to take part in next month's European Parliament elections.

The Commons has previously voted to show it does not want to leave the EU without a deal, which could indicate sufficient desire to pass the legislation.

But several lawmakers have expressed reservations about the unprecedented attempt to pass new laws without the government's approval. The constitutional concerns could sway some to vote against, or seek to stall the process.

The House of Lords is generally seen as being more pro-EU than the Commons, and it would seem likely that a majority exists to support the plan. However, the lack of controls on speaking time may mean opponents could filibuster it.

Legislative experts have so far expressed doubts about whether a piece of legislation proposed this week could be turned into law before April 10.