MPs have been told they face their “last opportunity” to ensure the Brexit process is extended to 22 May and bring certainty to people and businesses.
The attorney general said any further extension past 12 April would be “subject to the veto” of 27 EU leaders.
MPs will be asked to approve only part of the PM’s Brexit deal, the withdrawal agreement, covering the terms of exit.
Labour said separating that from the political declaration would mean voting on a “blindfold Brexit”.
Shadow Brexit Secretary Sir Keir Starmer told the BBC’s Today programme: “Take the political declaration off and it is completely blind – you have no idea what you are really voting for.”
MPs will vote on the withdrawal agreement, a legally-binding document setting out the terms of the UK’s departure from the EU, at 14:30 GMT.
Attorney General Geoffrey Cox told MPs the deal must be approved by 23:00 GMT if the UK wants to “secure our legal right” to an extension to Brexit.
He said that the European Council last week had agreed that Brexit could be delayed until 22 May – if the withdrawal agreement was approved by the end of this week.
If they do not, it offered a shorter delay until 12 April the date by which the UK would have to indicate whether it would stand candidates in the 2019 European Parliament elections allowing the UK time to get the deal through or to “indicate a way forward”.
“This is therefore the last opportunity to take advantage of our legal right and the government has taken the view that it would be wrong to allow that time and date to expire, without giving this House the opportunity to consider whether it should avail itself of the legal right,” Mr Cox told MPs.