Michel Barnier has raised the prospect of the Brexit transition period being extended until 2022, heaping pressure on one of the core messages of Boris Johnson’s election campaign.

Speaking on Tuesday the EU’s chief negotiator warned that the prime minister would face a “moment of truth” in the summer of next year and suggested that talks might not be concluded by then.

Cabinet minister Liz Truss had said on Monday night that “we will not be extending the Brexit transition period beyond 2020”, arguing that “the British people have waited long enough for Brexit”.

But the EU’s chief negotiator said on Tuesday that the UK would be facing yet another a no-deal cliff-edge at the end of 2020 if no agreement was completed.

“I know this negotiation will be difficult and demanding, for one reason the time will be extremely short, 11 months if the transition period ends as currently foreseen,” Mr Barnier told an audience in Lisbon.

“Summer 2020 in eight months will be the first moment of truth on how far we have come and whether extension of the transition will be needed.

“As long as we have not completed negotiations with the UK the risk of a cliff edge remains and we should all remain vigilant.”

Despite Mr Johnson’s claim that he would “get Brexit done” if voters give him a majority at the election, the UK in fact faces another no-deal cliff edge even if his deal is passed.

This is because if no free trade agreement has been secured by the end of next year, the UK would crash out without a deal.

During the transition period the UK abides by all EU rules with no say, and freedom of movement continues – a state of affairs that has enraged some Conservative Brexiteers, who described the situation as resembling a “vassal state”.

Despite the Government’s denials at every stage since the start of Brexit talks, Downing Street has always insisted it would never extend any deadline, but has always done so at every opportunity – casting significant doubt on the latest claims that it would make no further delay.

Under the terms of the withdrawal agreement, the UK can extend the transition period up to 2022 to make more time for trade talks, but has to make the decision in July, meaning the UK has eight months to close in on a trade agreement.

Liz Truss claimed the government would not extend the transition period (Jack Taylor/Getty) (Getty)

Asked why the Government had not taken the ability to extend the transition period out of the withdrawal agreement if they had no intention of using it, a senior UK government official suggested recently that negotiators were too busy dealing with the other issues in the treaty.

Summer 2020 in eight months will be the first moment of truth on how far we have come and whether extension of the transition will be needed. Michel Barnier, EU chief negotiator

Trade experts view the possibility of a deal being ratified in eight months as extremely unrealistic. Most free trade agreements take years to conclude, and holding to such a timescale would be unprecedented in a democratic nation.

Unlike the withdrawal agreement, EU free trade agreements often have to be approved by national parliaments, and in some cases regional parliaments – which demand concessions and are significantly more picky about their contents than national governments.

Mr Barnier also warned that there would be significant issues in pulling together any future trade agreement, telling his audience: “Our starting points are not necessarily the same.

“On our side the EU will require strong level playing field guarantees, the EU will not tolerate unfair economic competition.

“The UK should not think that zero tariff, zero quotas will be enough. The EU will insist on zero tariff, zero quotas and zero dumping.”

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Luisa Porritt MEP, a Liberal Democrat, said: “Barnier’s comments today show that there is no such thing as ‘getting Brexit done’ with Boris Johnson’s Brexit deal.

“Britain would face more prolonged uncertainty and a high risk of another no deal cliff edge when the transition period ends. The best option for Britain is to eject this appalling Tory Government from power by voting Lib Dem to stop Brexit and end this mess.”

Labour’s Brexit chief Sir Keir Starmer said the idea of a trade deal being negotiated in a few months was “for the birds”.