The EU has shot down Boris Johnson’s Brexit plan within moments of his appointment as Tory leader, in the latest sign that the bloc has no plans to make concessions.

In an intervention timed to coincide with Mr Johnson’s election announcement, Frans Timmermans, the European Commission’s first vice president, told reporters in Brussels that the EU would not renegotiate the deal reached with Theresa May.

Another EU commissioner, Vytenis Andriukaitis, also warned that politicians like Mr Johnson were undermining democracy with “cheap promises, simplified visions, blatantly evident incorrect statements”.

Mr Timmermans said: “He [Boris Johnson] took a long time deciding whether he was for or against Brexit and now his position is clear.

“I think the position of the EU is also clear: the United Kingdom reached an agreement with the European Union and the European Union will stick with that agreement. We will hear what the new prime minister has to say when he comes to Brussels.”

The bloc has said since last year it would not re-open talks on the withdrawal agreement struck by Ms May, which was rejected by MPs three times.

Mr Johnson has said he would try to use the withholding of payments owed to the EU as leverage to force it back to the negotiating table.

Mr Timmermans had previously suggested that Mr Johnson might not be sincere in his support for leaving the European Union, and could be “playing games”.

Asked about the comments on Tuesday, he said: “I would just suggest that you look at what he’s been writing over the years.

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“I don’t think I’m telling any secrets when I say that Boris Johnson took a long time to decide which side of the argument he was on. He did that publicly and in letters he’s written.”

Mr Timmermans, who is expected to stay in post after October and provide continuity between the current EU Commission and the next, added that the commission had “negotiated in good faith”, telling reporters: “I think if you have an agreement between a country and an organisation like the EU it’s the responsibility of all to stick to that agreement and make it work.”

He also suggested there was no mandate for a no-deal Brexit, adding: “I think if we look back at the last years I don’t think there were many people in the UK who, when they voted for Brexit, that intentionally voted for a no-deal Brexit. I don’t think that happened.”

In a separate blog post published on Tuesday, Mr Andriukaitis, Lithuania’s EU commissioner, also took aim at Mr Johnson and his “cheap promises”.

He wrote: “A functioning democracy demands discussion of us. Using WHATEVER means to win political battles just does not fit the bill. Boris Johnson ‘virtuoso’ in democracy is the example of this in action – where priority is given to the objective alone and not the means of obtaining it.

“On this shaky path almost anything is allowed: cheap promises, simplified visions, blatantly evident incorrect statements on ‘EU imposed’ food safety standards. Can democracy survive this type of politics? My take is that democracy chooses only those principles that derive from it, defend it and legitimize it. The ones that stem from ‘fake’ facts are killing it.

“Almost ironically, without comparing the UK itself with the USSR because it is not comparable, I can’t think of a better golden standard than the USSR in terms of fact distortion, reality falsification and blunt oblivions of reality.”

Boris Johnson begins by thanking Jeremy Hunt and Theresa May as he is announced as new Conservative Party leader

Mr Andriukatis added: “I can only wish him luck in ‘taking back control’, spending more money on the NHS, swiftly concluding new trade agreements.”

The commissioner also revealed that Jeremy Hunt, Mr Johnson’s opponent in the Tory leadership battle, had never responded to an invitation to discuss the differences between the EU and USSR. Mr Hunt had previously compared the two states, causing outrage in eastern Europe and elsewhere.

“Having spent some time of my life in the Soviet exile, I couldn’t help but offer to discuss the differences with him. Unfortunately, Jeremy Hunt did not reply to my call for lack of interest or otherwise,” he wrote.

EU chief negotiator Michel Barnier struck a more businesslike tone in welcoming Mr Johnson to office, however.

“We look forward to working constructively with prime minister Boris Johnson when he takes office, to facilitate the ratification of the Withdrawal Agreement and achieve an orderly Brexit,” he said.

“We are ready also to rework the agreed declaration on a new partnership in line with European Council guidelines.”

Incoming European Commission president Ursula von der Leyen also congratulated Mr Johnson.