FILE PHOTO: The Houses of Parliament form a silhouette against the late afternoon sky, in central London, Britain December 12, 2018. REUTERS/Phil Noble

LONDON (Reuters) - Several pro-EU lawmakers mounted a bid on Wednesday to force Britain’s government to return to parliament in three days with a plan B for Brexit if Prime Minister Theresa May’s deal to leave the European Union is voted down next week.

Lawmakers from the governing Conservative Party and opposition Labour Party have put forward a so-called amendment in parliament which would force it to offer alternatives to her deal in three days rather than the planned 21 days.

Some local media and lawmakers quoted sources as saying the speaker of parliament had accepted the amendment, meaning it would be voted on. The office of speaker John Bercow declined to confirm that he had accepted the amendment.

By reducing the timeframe for the government to come up with alternatives to the proposed deal, it puts additional pressure on May, who looks set to lose the vote on Tuesday in a move that would cast her Brexit plans into deeper uncertainty.

Parliament restarts a debate on her deal later on Wednesday after May pulled an earlier vote in December after acknowledging she looked set to be defeated.

First, lawmakers have to approve the so-called business motion, which sets out the procedure for the debate and vote on May’s deal. It is this motion that the lawmakers are seeking to amend.

Labour’s Brexit policy chief, Keir Starmer, said: “If the prime minister’s Brexit deal is rejected, parliament must decide what happens next. This amendment has Labour’s full support.”