LONDON - British Prime Minister Theresa May's Brexit deadline was Tuesday facing opposition from her own MPs.

May wants to enshrine in law that Britain and the EU will part company at precisely 23:00 hours on March, 29, 2019.

As MPs started their marathon debate on the EU Withdrawal Bill in the House of Commons, behind the scenes a number of Conservative MPs were meeting party managers to call for the government to scrap its strict timeline.

London-based newspapers reported that the number of MPs prepared to vote against the prime minister was in double figures, pointing to a humiliating defeat for May if they carry out their threat.

The Independent reported that former Education Secretary Nicky Morgan and another former government minister, Anna Soubry, were among the rebels.

Morgan, reported the Independent, said May's plan showed she was "tone deaf and tin eared", while Soubry told journalists she had warned May that there were some Conservative MPs who have never rebelled in their political careers but were now talking, for the first time ever, of rebelling against the government.

Former government attorney general Dominic Grieve said in the debate that setting the date of departure from the EU would hit the government's ability to carry out negotiations with Brussels.

The vote on the timeline is not taking place today, but Grieve said when the vote does take place he will vote against the government, adding that no arm twisting will persuade him otherwise.

Veteran politician and former cabinet minister Ken Clarke also criticized the government for tying down the date of departure to the second.