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
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