Boris Johnson’s Brexit negotiators received a sceptical welcome in Brussels on Wednesday during their latest trip to the EU capital to try to rewrite the Irish backstop.

There is growing frustration in the European Commission’s Berlaymont headquarters over a lack of concrete proposals coming from London, despite claims from the UK government that talks have been “intensified”.

EU officials are alert to the possibility that they may be, in the words of one, being “led up the garden path” by Mr Johnson’s team for the sake of giving the Tory leader an advantage in a coming election.

One theory getting a hearing in the Berlaymont is that Mr Johnson wants to keep talks frozen, with the possibility of both a deal and no deal open, because the ambiguity would suit him during an election campaign.

Officials have suggested that they will toughen up their rhetoric in the coming weeks if there is still nothing concrete from the UK – and do more to publicly call out the limited nature of discussions. Depending on when an election is called, such an intervention from Brussels could, significantly, land just in time for the campaign period.

Since the summer EU spokespeople and some member states have softened their rhetorical approach to the UK and stressed their willingness to engage with the new administration in Downing Street. But with the clock ticking down to a no-deal Brexit, patience is wearing thin.

When approached by reporters at Brussels’ main railway station, UK chief negotiator David Frost refused to answer questions about the nature of negotiations or give a statement. He is not expected to speak to the media during his trip.

An EU source with knowledge of the talks said Mr Frost’s team had last week come with “nothing, nada” in terms of concrete proposals to replace the backstop. UK negotiators apparently brought with them plans for how the Northern Ireland assembly could have a say on regulatory alignment for agricultural products between Northern Ireland and the Republic of Ireland – but not plans for the actual alignment itself.

However, Ireland’s EU commissioner Phil Hogan made positive noises about discussions earlier this week, suggesting that he remained hopeful that “the penny is finally dropping in London”. But one EU official close to negotiations suggested that the commissioner was being “a bit optimistic”.

In a live appearance on his Facebook page, Mr Johnson himself claimed he was "making great progress", adding: "I had some very good talks with Leo Varadkar in Dublin a couple of days ago, I've talked to Emmanuel Macron and Angela Merkel and the mood is changing, the ice-floes are cracking, there is movement under the keel of these talks and we can do this thing, absolutely.

"What we cannot do is fail to honour the commitment that all parliamentarians - or the vast majority of parliamentarians - made to the British people, and that is to come out of the EU on 31 October and not to extend Article 50."

A move by the UK government towards accepting agricultural alignment on the island of Ireland has raised some hopes that the UK position might be slowly moving back towards the Northern Ireland-only backstop proposed by the EU and rejected by Theresa May back in early 2018.

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But this is denied by the UK side, while EU sources are keen to stress that agricultural products are only one of many categories of goods needed to be covered by a backstop replacement, and that Britain would need to go much further.

Mr Johnson said: 'The backstop is going to be removed, I very much hope - or I insist - because that's the only way to get a deal. The UK parliament is not going to accept the current withdrawal agreement, there's no way that's going through.

"But the crucial thing to understand is that ... we will not accept a Northern Ireland-only backstop. That simply doesn't work for the UK. We've got to come out whole and entire and solve the problems of the Northern Irish border, and I'm absolutely certain we can do that."

Since Mr Johnson has come to office the UK has also moved to remove commitments to a “level playing field” from the political declaration on the future relationship – giving the UK government the option of running down environmental and social standards after Brexit.

"The UK is seeking to agree a free trade agreement. The EU have always said this is available. Any level playingfield provisions will need to reflect this end-state," one UK official said of the move.

Some EU officials suspect the hand of the United States in the policy change, given the requirements for a US-UK free trade agreement. They are also clear that without the level playing field, a UK-EU free trade agreement would be virtually impossible, because many member states would reject it.