Britain's opposition Labour Party said on Monday it would back calls for a second referendum on Brexit if parliament rejects its alternative plan for leaving the European Union.
With just over a month until Britain is due to leave the bloc on March 29, Prime Minister Theresa May is seeking changes to her exit deal in order to break an impasse in parliament.
Labour's decision could damage her hopes of winning support for a revised deal in a vote she has promised by March 12, by attracting those who would have backed her agreement to avoid a no-deal exit but who prefer a second referendum.
Parliament is due to debate and vote on Wednesday on the next steps in Britain's tortuous departure from the EU, and lawmakers are set to put down proposals, or amendments, which could include demanding the exit deal is put to a public vote.
Labour said it would put forward an amendment calling on the government to adopt its Brexit proposals, which include a permanent customs union with the EU and close alignment with the bloc's single market.
"If Parliament rejects our plan, then Labour will deliver on the promise we made at our annual conference and support a public vote," Labour's Brexit spokesman Keir Starmer said on Twitter.
Parliament is not expected to back Labour's Brexit plan.
Labour said it would support a bid by its lawmaker Yvette Cooper to give parliament the legal power to force May to delay Brexit by seeking an extension to the Article 50 negotiating period.
"We are committed to also putting forward or supporting an amendment in favour of a public vote to prevent a damaging Tory (Conservative) Brexit being forced on the country," Labour leader Corbyn was due to tell a meeting of his lawmakers on Monday, his office said.
"One way or another, we will do everything in our power to prevent no deal."