Boris Johnson and Jeremy Hunt’s Brexit plan to axe the Irish border backstop from the withdrawal agreement will be rejected outright by the European Union, EU sources have said.

Informed sources say that it is doomed to failure and if the next prime minister goes to Brussels with such a plan, he will be told in “no uncertain terms” that it amounts to a declaration of no deal.

Brussels had already rebuffed such a plan when the Brexit secretary, Steve Barclay, who is now part of Johnson’s campaign, met the EU’s chief Brexit negotiator, Michel Barnier, last week.

In what was seen as “spinning for a Boris plan”, Barclay told Barnier that the backstop was dead five times during the meeting. Sources say he told Barner that they wanted a series of mini deals and alternative arrangements for the Irish border.

He was told that was Brexit fantasy and a non-starter, and that the “mini deals” outlined in EU contingency plans were temporary and only covered the “bare bones” such as aviation, mobile phone roaming and haulier driver licences. They did not include the major issues such as trade or the Irish border.

The EU is watching developments in the UK very closely and has already prepared responses on a range of possibilities including a call for the EU to endorse the so-called “Brady amendment” which was passed in the House of Commons in January. It called for the backstop to be scrapped.

This will also be rejected.

Johnson and Hunt have declared the Northern Ireland backstop “dead” and promised to throw it out of any deal they negotiate with the EU, in comments that significantly harden their Brexit positions.

While their words may be the source of alarm, the Irish fully expected this and see it as campaign spin.

Leo Varadkar, the taoiseach, said in a radio interview over the weekend that he would give whoever became prime minister “a fair hearing” but warned that the victor would be in for a “reality check” when he got the keys to Downing Street.

“Politicians when they are in campaign mode, and both of those men are in campaign mode, tend to campaign in poetry, in simple terms and high-level messages.

“When you get into office you have to govern in prose, and I imagine whoever is the new prime minister is going to face a very serious reality check when they sit down with their officials,” he told Pat Kenny on Newstalk.

On Monday night, the Tory leadership rivals both ruled out trying to tweak the backstop, which critics say could trap the UK indefinitely in a customs union with the EU.

The EU is watching carefully and is expecting that if Johnson, the frontrunner, is selected, he will make immediate plans to fly to Brussels and to Dublin for talks.

If he decides to make his first visit the US, this will “embolden” EU member states because it will send a clear signal that he deems the US more important than the EU, the UK’s biggest trading partner.

There is an expectation that Johnson and his negotiators will come to Brussels in September with an early outline of his new Brexit plan. But EU sources warn that it will have to be tested in parliament first because they will not waste time negotiating anything that does not have a mandate in the House of Commons.

The new UK prime minister might seek a string of protocols on alternative arrangements that would guarantee the backstop would never be used.

“That would mean an insurance policy that would never be used. There would be no point to that,” said an EU source.

Varadkar said in a lengthy interview with Kenny that Ireland was prepared for no deal but he hoped it would not happen.

“It will be incredibly severe on Northern Ireland because it will face tariffs … and will face huge difficulties, because a huge amount of trade from Northern Ireland goes through Dublin.

“The impact on Northern Ireland will be more severe than anywhere in Europe. The impact on the UK and Ireland would be pretty severe too. I hope a new British prime minister wouldn’t willingly do that,” he said.

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