EU chief Brexit negotiator Michel Barnier has said British Prime Minister Theresa May's proposals for a backstop customs arrangement in Northern Ireland raise a series of "difficult" questions.

He said it was not necessarily "feasible" to extend the EU's offer of continued participation in key elements of the customs union in Northern Ireland to cover the whole of the UK, as proposed by London.

And he said Mrs May's insistence that the arrangement must be time-limited meant that it could not be regarded as a true backstop, providing a fallback option if the UK's preferred permanent solution could not be agreed.

"Backstop means backstop," he said. "The temporary backstop is not in line with what we want or what Ireland and Northern Ireland want and need."

Mr Barnier said it appeared that some Brexit supporters wanted to offload on to Brussels the blame for the fact that the UK cannot continue to enjoy some of the benefits of EU membership after leaving.

But he said: "We are not going to be intimidated by this form of blame game."

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Meanwhile, a spokesman for Mrs May said Britain will never accept a customs border between Northern Ireland and the rest of the United Kingdom and that position will not change, a government spokesman has said.

"The prime minister has been clear that we will never accept a customs border between Northern Ireland and the rest of the United Kingdom. We are also committed to maintaining the integrity of our own internal market. That position will not change," the spokesman said.

"The Commission's proposals did not achieve this, which is why we have put forward our own backstop solutions for customs."

Under the proposals thrashed out by Mrs May in tense meetings with Brexit-backing ministers David Davis and Boris Johnson on Thursday, the whole of the UK would remain part of key elements of the customs union until a better arrangement is in place - something which the Government expects can be achieved by the end of 2021.

Mrs May believes this would keep the Irish border open, while avoiding creating a customs border between Northern Ireland and the British mainland, as the EU's backstop proposal would.

But Mr Barnier said: "Our backstop can't be extended to the whole UK. Why? Because it has been designed for the specific situation of Northern Ireland."

Under the EU proposal "Northern Ireland would form part of our customs territory", he said, adding: "What is feasible with a territory the size of Northern Ireland is not necessarily feasible with the whole UK."

And he questioned whether Mrs May's proposals would deliver an "all-weather backstop" fit for all circumstances.

"The UK calls this arrangement 'temporary'," said Mr Barnier.

"How does that fit with the need to secure the absence of a hard border in all circumstances?

Mr Barnier warned that "time is moving on", with less than three weeks to go to the 28 June EU leaders' summit at which Brussels and the Irish Government are pushing for resolution of the border issue.

"The time has come today to take decisions and make choices," Mr Barnier said.

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UK plan a 'small step forward' - Varadkar

Taoiseach Leo Varadkar has said the British government's temporary plan to avoid a hard border between Ireland and the UK represents a "small step forward".

Speaking in Belfast, Mr Varadkar said there are three questions to ask of the paper: "does it achieve what we want it to achieve - that there is no hard border? Is it all-weather, does it work in all circumstances? And does it respect the integrity of the single market and the customs union?"

He said: "My initial view is that it is a step in the right direction, it is welcome but it does fall short ... because it deals with the customary aspect of the border rather than the regulatory aspect.

"In fairness to the government in London, they accept that and they make it clear that this is only about the customs element of the backstop and we do have a difficulty with any sort of deadline.

"The only deadline that should be in the backstop is the all-weather or if and when deadline. So the backstop should apply until such a time as there is an alternative arrangement, a new EU-UK relationship which avoids a hard border.

"So just putting off a hard border for two, three, six or 20 years is not enough. It has to be permanent.

"What we have now is something that we can work and negotiate around. That is something we have not had for the last two years so it is a step in the right direction. It is welcome. Even it does fall short." 

Mr Varadkar said that Ireland had to put the UK under a lot of pressure and stand united with its European allies to make changes happen and now we have seen another change in position.

He said that when the referendum was passed in the UK, many people had not thought through what Brexit meant and what a new relationship would look like and that now they are really struggling.

In a statement, Nigel Dodds of the DUP said Michel Barnier's latest comments demonstrate that he has no respect for the principle of consent of the constitutional integrity of the United Kingdom.

Mr Dodds said: "this is nothing more than an outrageous attempt to revert to the annexation of Northern Ireland. We will not accept such a proposal."

He said Mr Barnier fails to understand that both the Labour Party and the Conservative Party have already indicated that his proposed backstop is not acceptable.

"No Prime Minister could ever agree to any arrangement which would threaten the economic and constitutional integrity of the UK," he said.