Theresa May has been forced to defend her Brexit plans amid claims the EU is likely to reject them.

The Prime Minister insisted proposals outlined in a major speech on Friday to maintain an open border in Northern Ireland and ensure a close trading relationship with Europe were “credible”.

It comes as European leaders suggested the plan would be difficult to accept because it would undermine the EU’s single market.

Ireland’s foreign minister, Simon Coveney, was among those suggesting the EU may reject the plan.

He told the BBC’s Andrew Marr Show: “She hasn’t really gone into any more detail than we’ve already heard in terms of how she’s going to solve the problem of maintaining a largely invisible border on the island of Ireland.”

Mr Coveney said he was “not sure that the European Union will be able to support” the UK’s plan because it poses a threat to “the integrity of the single market”.

He said: “When somebody from the British Government definitively says that something will be the case, people assume that that is the negotiated outcome. Of course it’s not.

“While of course we will explore and look at all of the proposed British solutions, they are essentially a starting point in negotiations as opposed to an end point.”

In a major speech in London on Friday, Ms May confirmed the UK will be leaving the customs union and single market but suggested an open border could be maintained in Northern Ireland through technological solutions and ensuring similar regulations are maintained between Britain and the EU.

Mr Coveney said: “This isn’t a question of either side wanting to put up borders, but if you have to protect a functioning single market, just the same way Britain wants to protect its own single market, then you have to understand that if goods move from one customs union to another then there needs to be some checks, unless there is some mechanism that is negotiated and put in place that prevents that.”

If negotiations fail to reach an agreement then the “backstop” plan of full British alignment with the customs union and single market would have to be put in place, he said, insisting that the UK Government had “committed clearly” to this last year.

Labour peer Lord Mandelson, a former European Commissioner, also claimed the EU would never accept the plan “in a month of Sundays”.

He said: “What Theresa May is doing is trying to dance on the head of a pin that simply doesn’t exist.”

“It will be painful for the country as a result.”

Responding to claims her proposals were unrealistic, Ms May insisted her speech contained “credible” proposals for maintaining an open border in Northern Ireland.

“We’ve got proposals as to how we’re going to achieve that – now we’re going to be able to sit down and talk with others about how we’re going to do that,” she told Andrew Marr.

“This is an ambitious deal but what I’ve put forward is credible because it is based in practicality.”

She denied the plan amounted to “cherry-picking” saying: “If you look at the European Union today, it has different trade agreements with different countries around the world.

“If you say that looking at what suits your particular economies and putting that into a trade agreement is cherry-picking, then they are cherry-picking in every trade agreement they put forward.”

In the first signs of Conservative dissent over Ms May’s speech, Tory grandee Lord Heseltine said it proved the Government had “no answer” on how to implement Brexit.

He told The Observer: “The speech just moves us further down the cherry-picking road. It set out the cherries that Britain wants to pick but that approach completely ignores the fact that the EU has said, ‘sorry, there is no cherry-picking’.

“Why is it that after 18 months since the referendum we have not got any closer with these issues? The answer is simple: because no one has got any answer about how to do it.”

However, Ms May was given a boost when a senior Tory rebel said the speech had given the Prime Minister “breathing space” on a key vote that could force her to change her Brexit policy.

Pro-EU Tory MPs believe they have enough support to defeat the Government by passing an amendment to the Trade Bill that would force ministers to keep the UK in a customs union with the EU.

But in a sign of backbench dissent falling away, Sarah Wollaston, who has backed the amendment, told ITV’s Peston on Sunday: “I think what we need to see is some kind of partnership that allows frictionless trade at our border and also deals with the situation in Northern Ireland and I want to give her the space to do that.’’