Brexit implementation period could go a few months either way in 2021 - Damian Green

He also refused to rule out paying for continued access to the European single market after leaving the bloc.

Mr Johnson used an eve-of-conference interview with the Sun to insist that the implementation period should last "not a second more" than two years and there should be no ongoing payment for single market access after Brexit.

The move caused fresh controversy and overshadowed the start of the Conservative Party conference, with critics claiming Mr Johnson should face the sack for seeking to impose conditions on the Prime Minister.

Asked what an appraisal of Mr Johnson's ministerial performance might say, Mr Green said: "Boris is doing what Boris has always done - adding to the gaiety of nations."

At a fringe event at the conference in Manchester, Mr Green said the implementation period could go beyond two years for some parts of the economy.

He said: "The phrase the Prime Minister used was 'around two years' but that means a few months either way."

He added that could mean "instead of the end of March it's the end of June" 2021 and the Government was not committing to two years "to the minute".

Mr Green also refused to rule out annual payments for ongoing single market access - indicating that he did not want to constrain Brexit Secretary David Davis during the negotiations.

"I'm not ruling anything in or out that will be in the detail of the negotiation because David Davis has got a difficult job and he is eminently suited to doing it," he said.

"I'm not going to do anything that makes his job more difficult."

Mr Green played down the prospect of leaving the EU without a deal - despite Mrs May's stance that "no deal is better than a bad deal".

Leaving the EU and resorting to World Trade Organisation terms "would be much less good than having a decent agreement" for both sides, he said, but stressed a deal was likely to be reached with Brussels.

"If you study European negotiations, they follow the same pattern where there's a lot of posturing, a lot of people saying a deal is completely impossible, and then at the last minute a deal is done.

"It is overwhelmingly almost always the case in the EU that a deal is done."

Trade talks with other countries would be able to take place during the implementation period, but deals could not be signed until it expired, he suggested.

The role of the European Court of Justice is a potential stumbling block in the Brexit talks, and Mr Green stressed that there "won't be direct control" by the Luxembourg court in the way there is now.

But there would need to be a "dispute resolution procedure" governing the deal, which he suggested could involve a panel made up of British and EU judges.