In an ornate reception room at the Irish Embassy in London on Wednesday evening, a distinguished assembly of MPs, peers, senior Whitehall officials, academics and diplomats milled around, sipping drinks and nibbling canapés. Such high-level gatherings are regular events at the Embassy, but this one was different, charged with the kind of tension that has become unfamiliar in recent years.

Ambassador Adrian O’Neill acknowledged that the British-Irish relationship was going through one of its periodic “stresses and strains”. But nobody could remember the last time relations between the two countries were so strained or so stressful.

As Britain’s negotiations with the European Union enter a crucial period ahead of next month’s summit, Ireland has emerged as the greatest obstacle to progress. Theresa May has dramatically increased her financial offer to the EU in the hope that the summit will agree that “sufficient progress” has been made to move to the second phase of talks – about the future trading relationship between Britain and the EU.

Taoiseach Leo Varadkar and Minister for Foreign Affairs Simon Coveney insist, however, that Britain has not made sufficient progress on outlining exactly how it proposes to avoid the return of a hard Border in Ireland. Ireland’s position, the substance of which has not changed, has met with surprise in London and with outrage in parts of the media.

“The Sun has some advice for Ireland’s naive young prime minister: shut your gob and grow up,” the newspaper wrote in an editorial last week.

“Leo Varadkar may not like Brexit. So what? He needs to accept it’s happening. His priority should not be picking holes in our position. It should be helping make Brexit work for millions of his citizens and ours, including by engaging constructively on a Border solution.”

The Spectator this week described mounting anger within the British government over Ireland’s position, quoting one cabinet minister warning that Varadkar was “playing with fire”.

Conspiracy theory And the BBC’s political editor, Laura Kuenssberg, reported a conspiracy theory doing the rounds in Whitehall and Westminster. According to the theory, Ireland and the EU are over-egging their fears about the Border so they can use the issue later in the negotiations to Britain’s disadvantage.

The analysis of Ireland’s position may be misleading and the surprise unjustified, but the anger in London is real because the stakes are so high. If the EU does not agree to move to the second phase of negotiations next month, the consequences for May could be enormous.

Irish officials are right when they say the Government’s position on the Border has not changed since last year’s referendum, and that Enda Kenny was asking for exactly what Varadkar is looking for. But there is no denying that the message sounded more emollient when it came from Kenny, whose personal manner could scarcely be more different from Varadkar.

Socially awkward in different ways and both averse to small talk, Varadkar and May have no personal chemistry. But it was Coveney who set the sharp new tone on Brexit last July, during a series of private meetings in London.

During the meetings he made plain Ireland’s unhappiness about Britain’s approach to the Border, dismissing the role of technology in keeping it frictionless and demanding a political solution. Among the solutions he floated was extending the EU Customs Union to include Northern Ireland after Brexit.

Change in tone A week later the London Times ran a story saying that Coveney was “pushing for the Irish Sea to become the post-Brexit Border”, reporting that British officials were “taken aback” by the change in tone.

“There is a new Taoiseach and a new Foreign Minister, and they’re stamping their authority. We’re being as positive as we can, but it’s true to say their attitude has hardened,” a senior British government source told the paper.

If the purpose of the leak was to undermine Coveney’s robust approach it failed. Varadkar not only endorsed his Foreign Minister’s line, which did not in fact call for a Border in the Irish Sea, but amplified it.

May’s position is complicated by her government’s reliance on the votes of the DUP’s 10 MPs at Westminster. Since their success at June’s general election, which followed an Assembly election a few months previously, influence within the DUP has shifted from Stormont to Westminster. Although the DUP as a whole backed Brexit, the party’s MPs tend to be more ideologically committed to leaving the EU.

DUP MPs privately express bewilderment at Dublin’s approach, arguing that, since avoiding a hard Border is one of the few issues on which all parties on the island agree, they should work out a practical solution together.

Outrage Some of this week’s outrage in Britain was triggered by the Taoiseach’s remarks on the way in to last week’s summit in Gothenburg, Sweden.

“It’s 18 months since the referendum, it’s 10 years since people who wanted a referendum started agitating for one. Sometimes it doesn’t seem like they thought all this through,” he said.

The statement, which sounded like common sense in Dublin, came across in Britain as arrogant and disrespectful.

Brexiteers are slow to accept invitations to debate the issue with Irish groups because of bruising experiences of being harangued and told how foolish their decision in the referendum was and how little they understand the EU.

Ahead of next month’s summit, the Irish Government wants Britain to make a written declaration outlining how it plans to avoid a hard Border.

“Every detail cannot be solved at this stage, but what is needed is a firm commitment from the UK that the final outcome will maintain the openness and invisibility that characterises the Border today with its 300 crossings along 300 miles, and that such an outcome will respect Ireland’s position and related responsibilities as an EU member-state,” the Ambassador said on Wednesday.

Financial offer British officials are hoping that Ireland’s EU partners will tell Varadkar that they cannot allow the Irish issue to hold up progress, particularly in view of Britain’s substantial financial offer. There are signs that a compromise might be possible, perhaps in the form of a British declaration and an EU commitment to defend Ireland’s interests in the second phase of the negotiations.

The reception at the Embassy this week was for the launch of a new paper by Irish academic Edward Burke on the impact of Brexit on Northern Ireland.