LONDON A number of members of parliament who opposed a British exit from the European Union in June's referendum would now back a start of formal divorce proceedings from the bloc - provided parliament gets to decide at all, a Reuters poll showed.
Results of the online survey opened the possibility that Prime Minister Theresa May might be able to win a vote in what has been a predominantly pro-EU parliament, although her government remains determined to prevent such a vote from happening.
May has said she will trigger Article 50 of the EU's Lisbon Treaty - which begins an initial two-year period during which Britain must negotiate the terms of its exit - by the end of March next year without giving MPs a vote.
However, London's High Court is due on Thursday to start hearing a legal challenge led by a pro-EU investment fund manager seeking to force May into letting parliament decide when, how and whether to invoke Article 50.
In the online poll, Reuters asked members of parliament's lower House of Commons - excluding the nearly 100 who hold government posts and are therefore obliged to follow May's line - how they would vote if the challenge were to succeed.
Of the 57 MPs who responded, more than 60 percent said they would back the start of formal negotiations. Over a third of respondents who had voted "remain" in the June 23 referendum said they now supported triggering the Brexit process.
"The consequence of a leave vote was clear to everyone, that we would leave the EU. The majority of remain voters do not want the democratic process in the referendum bypassed by MPs," one Conservative who supported remain but now favours triggering Article 50, said in response to the anonymous survey.
May has described legal challenges to Brexit - another is underway in Northern Ireland - as an attempt to "subvert" democracy and delay the process after Britons voted by 52 to 48 percent to leave the bloc.
In the poll, all 21 MPs who voted leave in June supported invoking Article 50. Of the 36 respondents from the remain camp, 14 said they had come round to backing a start to Brexit negotiations.
May would need a significant shift in opinion among MPs, were it to come to a parliamentary vote. Around three-quarters of the 650 members of the lower house backed remaining in the EU before the referendum. Many say they would now respect the wishes of the country and not block the exit.
However, May's Conservatives have long been deeply divided over EU membership and she herself voted in June to remain. The party has a small majority in the lower house and none in the upper House of Lords, so parliament's involvement could risk seeing the Brexit process prolonged by months or even years.
Attorney General Jeremy Wright will represent the government in this week's London court proceedings, and will argue that a decision to withdraw from the EU is a power that only ministers can take on behalf of the monarchy.
Lawyers expect the final decision will be made by the Supreme Court, Britain's highest judicial body, in December.
Victory for the claimants would mean parliament would have to debate triggering Brexit, vote on Article 50 and then pass legislation, a process which could delay an EU exit.
The nearly 100 Conservative MPs who hold government roles would back the government position if a vote took place. Only one of the 26 Conservative MPs who responded to the survey said they would vote against triggering Article 50.
The opposition Labour Party has yet to reveal its official position if a vote were held, but a third of the 24 Labour MPs who responded to the survey said they would back triggering Britain's EU exit.
Even if the House of Commons backs the divorce proceedings, the government may run into trouble in the House of Lords, where it has had several measures blocked over the last year.
Some MPs said they would not support triggering Article 50 until the government had set out more about its negotiating position and what kind of future relationship it was seeking with the EU. Others said parliament should be given a vote on the terms of the final Brexit deal.
"It seems that the government want to draw up negotiating terms, negotiate and reach a deal without any parliamentary approval. That is not making parliament sovereign, it is sidelining parliament," Labour's Brexit spokesman Keir Starmer told parliament on Monday.
Labour wants a vote on the basic terms proposed by the government before Article 50 is invoked, he said.
"We accept and respect the result of the referendum, but neither those who voted to remain nor those who voted to leave gave the government a mandate to take an axe to our economy."
(Editing by David Stamp)