LONDON (Reuters) - Prime Minister Theresa May said on Wednesday she would permit a delay to Britain’s departure from the European Union in exceptional circumstances, bowing to criticism from her own party over the government’s plan to fix the exit date in law.

The decision is a compromise with Conservative lawmakers who last week rebelled in parliament and inflicted an embarrassing defeat on May during a debate on the legislation that will end Britain’s EU membership.

The legislation, formally titled the European Union (Withdrawal) Bill, later won approval to move to the next stage of the parliamentary process, although it still faces weeks of further scrutiny before becoming law.

May headed off a second rebellion by agreeing that her government’s plan to define the date of Britain’s EU exit as March 29, 2019, should be tempered by inserting a proviso allowing that date to be changed if necessary.

“If that power were to be used, it would be only in extremely exceptional circumstances and for the shortest possible time,” May told lawmakers. Parliament will have to approve any new date.

Junior Brexit Minister Steve Baker added that he could not envisage the date being brought forward.

TIGHT TIMETABLE

The passage of the bill to the next stage was overshadowed by the resignation of May’s most senior ally in government, Damian Green, at May’s request after an internal investigation found he had breached the government’s code of conduct.

The destabilising resignation adds to the political difficulties May faces as she tries to deliver Brexit against a backdrop of a divided parliament and electorate, and questions about her ability to meet an already tight timetable.

She wants to negotiate a transition deal with Brussels by March to reassure businesses and then seal a long-term trade deal by October. Brussels has said a detailed trade deal is likely to take much longer, and that Britain’s transition period must end by 2020.

In addition, May’s government has to undertake the huge legislative task of transferring the existing body of EU law into British law before it leaves in order to provide legal certainty for businesses.