LIVERPOOL, England - Britain's main opposition Labour Party is set to vote down any deal Prime Minister Theresa May clinches with the European Union and is open to a second referendum with the option of staying in the bloc, its Brexit spokesman Keir Starmer said on Tuesday.

With just over six months until Britain leaves the European Union, May has yet to reach a deal with Brussels on the terms of the divorce, and her plan for future trade ties has been rebuffed by both the EU and many lawmakers in her own party.

Labour has listed six tests it would apply to any Brexit deal, including whether it ensured a strong future relationship with the EU and delivered the same benefits Britain has as a current member of the bloc's single market and customs union.

Starmer said May was on course to fail these tests.

"Everybody recognises the talks are going badly and it looks as though we're heading for a bad deal or even no deal," he told BBC TV. "We, the Labour Party, are going to vote down a bad deal or we're going to vote down no deal because that is not good for our country nor is it what people voted for."

In a speech to his party's annual conference later, Starmer will say the Conservative government does not have a credible plan for Brexit, and that there is no majority in parliament for May's so-called "Chequers" proposals, which envisage close ties with the EU in the trade of goods.

Labour could play a decisive role in whether any Brexit deal is approved by parliament. May has a working majority of just 13 in the 650-seat parliament and a former junior minister said this month as many as 80 of her own lawmakers were prepared to vote against a Brexit deal based on the Chequers proposals.

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Labour's conference will vote later on Tuesday on keeping a second Brexit referendum as an option if May fails to get her Brexit plan through parliament, heaping pressure on the struggling prime minister.

Starmer said a meeting of party officials on Sunday had agreed that any second vote would allow for Britons to vote to stay in the EU after all, seeming to contradict the view expressed by the party's finance spokesman on Monday.

"The question that would be asked was left open because we don't yet know the circumstance we'll find ourselves in," Starmer said. "The meeting on Sunday was very clear that the question would be wide enough to encompass the option of remain. Nothing is being ruled out, including the option of remain."

The government's Brexit Secretary Dominic Raab said on Monday that Labour's "nonsense" about a second referendum would encourage the EU to offer a "lousy" deal and most people in Britain just wanted politicians to get on with Brexit. 

"Labour seem determined to take us all back to square one by rejecting a deal out of hand then trying to delay Brexit and re-run the referendum," junior Brexit minister Robin Walker said in a statement.

"Labour promised to respect the referendum result, but are just playing political games and trying to frustrate it."

(Additional reporting by Michael Holden; Editing by Alison Williams) ((kylie.maclellan@thomsonreuters.com; +44 207 542 0401; Reuters Messaging: kylie.maclellan.thomsonreuters.com@reuters.net))