LONDON (Reuters) - Prime Minister Theresa May was under growing pressure on Sunday to change her plan for Britain to leave the European Union to avoid defeat in a parliamentary vote.

FILE PHOTO: Steve Baker, a Minister at the Department for Exiting the European Union, leaves Downing Street, in central London, Britain June 14, 2017. REUTERS/Peter Nicholls

With both Britain and the EU suggesting an agreement is close, eurosceptic lawmakers and a leading member of a small Northern Irish party that props up her Conservative government made new threats to vote against the terms of the deal she is working on with Brussels.

The vote in parliament, most likely to come later this year, is gearing up to be the biggest showdown in the lengthy negotiations to leave the EU, Britain’s biggest shift in foreign and trade policy in more than 40 years.

May, who was attending a ceremony to mark 100 years since the end of World War One, found some support from ministers in her cabinet, but it would be hard for her to ignore the growing calls to change tack after a minister resigned and Northern Ireland’s Democratic Unionist Party threatened to rebel.

“If the government makes the historic mistake of prioritising placating the EU over establishing an independent and whole UK, then regrettably we must vote against the deal,” Steve Baker, a leading eurosceptic and former minister, wrote alongside the DUP’s Brexit spokesman Sammy Wilson in the Sunday Telegraph newspaper.

The main battleground is over a so-called backstop to prevent the return of a hard border between the British province of Northern Ireland and EU-member Ireland, something that would only come into force if a deal on future ties cannot guarantee the type of frictionless trade needed to keep it open.

DEPTH OF ANGER

Fears that proposals would mean keeping Britain inside the EU’s customs union indefinitely or that Northern Ireland would have to accept different rules and regulations to the rest of the United Kingdom have focused opposition to May’s deal.

Eurosceptics have long criticised May’s proposals, but it was the resignation of Jo Johnson, the remain-voting younger brother to Brexit campaign leader Boris Johnson, that highlighted the depth of anger over her plans.

Many say May’s desire to prioritise free flowing trade of goods with the rest of Europe will make Britain little more than a “rule taker”, unable to break free of Brussels’ decisions.

Comments by Education Minister Damian Hinds will also do little to temper those concerns. He told the BBC Britain could not push for a deal giving London the sole right to say when it was ready to leave any backstop arrangement.

“The prime minister has to negotiate something which is negotiable with the other side as well as working for people here,” Hinds said.

But May did find some support. Both he, and the eurosceptic leader of the lower house of parliament, Andrea Leadsom, said they supported the prime minister.

“I mean to support the prime minister to get a Brexit that works for the United Kingdom and the EU, keeps our country together and delivers on the referendum,” said Leadsom, who some media reported was considering quitting her post.

She said she did not expect further resignations, adding: “I do urge colleagues to support the prime minister. We are at a very difficult stage ... but it is a negotiation, so what we have to do is to hold our nerve and keep negotiating.”