Boris Johnson is seen responding to comments on his statement on Brexit deal in the House of Commons.

The British parliament may end up forcing Prime Minister Boris Johnson to request another Brexit delay and give only provisional support to his divorce deal with the EU.

An amendment penned by former government minister Oliver Letwin forces Johnson to write a letter to Brussels requesting an extension of the Brexit deadline until January 31 of next year.

Johnson is currently required to ask for an extension if parliament on Saturday votes down the revised withdrawal agreement he signed with EU leaders in Brussels this week.

But Letwin's amendment, which was selected by House of Commons Speaker John Bercow, would decline to approve Johnson's deal in full "unless and until" every other part of the formal Withdrawal Agreement Bill is passed.

Johnson hopes to introduce the formal bill next week once MPs have voted on Saturday's political agreement.

Letwin says he backs Johnson's agreement but is concerned that lawmakers might fail to push through the required Brexit implementation mechanism by the October 31 deadline.

This would, theoretically, result in a "no-deal" split at the end of the month, a scenario Letwin's amendment is designed to avoid.