Efforts to persuade MPs to back Theresa May's Brexit deal will continue on Thursday, a day after she promised to quit as PM if it was approved.

Her pledge brought some on-site, such as ex-foreign secretary Boris Johnson.

But challenges remain for the PM after Northern Ireland's DUP, who she relies on for support, said it would not back the deal because of the Irish backstop.

Meanwhile, none of eight alternative Brexit proposals brought by MPs secured backing in a series of Commons votes.

The options - which included a customs union with the EU and a referendum on any Brexit deal - were supposed to help find a consensus over how best to leave the EU.

But the failure of any of them to garner support from a majority of MPs led Brexit Secretary Stephen Barclay to say it strengthened ministers' view their deal was "the best option".Conservative MP Sir Oliver Letwin, who oversaw the unprecedented process of "indicative votes", said the lack of a majority for any proposition was "disappointing".

But he told BBC Radio 4's Today programme no "assumptions" should be made about the outcome of further indicative votes, which he believes should take place on Monday if the PM's deal is not approved this week.

"It's very difficult to translate from how people vote the first time when they don't know how other people are voting, to how they will vote when they can see how other people are voting under new circumstances," he said.

Ahead of Wednesday's debate, Mrs. May told a meeting of Conservative backbenchers she would leave office earlier than planned if it guaranteed Parliament's backing for her withdrawal agreement with the EU.

Mrs. May told her MPs: "I have heard very clearly the mood of the parliamentary party. I know there is a desire for a new approach - and new leadership - in the second phase of the Brexit negotiations - and I won't stand in the way of that."

She told MPs she would resign as party leader after 22 May - the new Brexit date - but stay on as PM until a new leader is elected. However, Downing Street said it would be a "different ball game" if the deal was not passed.

Her announcement that she would not lead the talks with Brussels over the future relationship between the UK an EU prompted a number of Tory opponents of her deal to signal their backing.

Prominent Leave supporters such as Mr. Johnson and former Tory leader Iain Duncan Smith said they now viewed the deal as the least-worst option.