Dominic Raab has been accused of engaging in diplomatic “sabre-rattling” after rejecting legislative alignment with Brussels and dismissing claims by Michel Barnier that the Brexit agreement makes customs checks in the Irish Sea “indispensable” .

The UK foreign secretary said following EU rules after 2021 “just ain’t happening” and insisted it was wrong for the EU’s senior negotiator to claim that goods entering Northern Ireland and Great Britain would be subject to extra paperwork once Britain leaves the post-Brexit transition period.

The UK is embarking on an 11-month transition period during which Boris Johnson and his government will try to secure a future trading and security relationship with the EU. If there is no deal by 31 December, the UK will face a “cliff-edge” no-deal Brexit.

Asked on Sky News on Sunday if Michel Barnier was wrong to say there would be border checks on goods moving between the EU and the UK, Raab said this was up to the EU.

“Yes, he is wrong if the EU lives up to its commitments on its side both in the withdrawal agreement and the political declaration,” he said. “We expect those assurances and agreements to be kept to. That’s why we’ve done this deal – it is a package.”

Barnier made the claim in a speech at Queen’s University in Belfast earlier this week when the EU negotiator said the withdrawal agreement was clear on checks. “The text is very precise. I always tell the truth,” he said.

Later, Raab told the BBC’s Andrew Marr Show that following EU rules after 2021 “just ain’t happening”.

“We are entering into these negotiations with a spirit of goodwill. But we are just not doing that other stuff. The legislative alignment, it just ain’t happening,” he said.

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Foreign Secretary Dominic Raab: “It’s not even a red line. This is not on the table”https://t.co/ukkyyYLfLf pic.twitter.com/ubRgWmvFpz

He accused Brussels of attempting to “shift the goalposts” since the political declaration was signed off last year.

Asked about reports of EU demands for the European court of justice to oversee disputes in any trade deal with the UK, Raab said: “We all agreed that we weren’t going to do that. You knew that, you signed up to the withdrawal agreement and political declaration on that basis. Why are you trying to shift the goalposts?

“We want to have a good positive win-win new deal with the EU. That’s not going to happen if they pull the rug, shift the goalposts.”

John McDonnell, the shadow chancellor, said Raab’s words were a showy and childish attempt to set out the government’s position in the trade negotiations. “It is ‘sabre-rattling’ … It is a bit puerile,” he told the Andrew Marr Show.

Raab’s claims followed reports that British diplomats have been ordered to make a symbolic break with Brussels.

After Johnson set out a hardline position before talks with the EU begin, the Irish prime minister, Leo Varadkar, said the EU could reach a sector by sector deal with the UK if a comprehensive Brexit agreement was not reached by Johnson’s December deadline.

But he told the BBC it would be a “second-class outcome” and mean that negotiations could drag on for years.

“If it isn’t possible to have a comprehensive deal then the back-up plan would be sectoral agreement or some temporary agreements that would buy us more time while we negotiate the comprehensive agreement, but that to me is an inferior outcome. And imposition of bureaucracy, checks, tariffs and quotas, that is not good for anyone,” he told Marr.

He added that such a deal “by its nature would be temporary or short-lived because we would still have to come to the agreement on the other matters, which means everything running on to 2021 and 2022”.

It is understood Downing Street is not keen on a sector by sector agreement but it could be a fallback that would allow pacts on vital national interests such as security and aviation if talks on other issues break down.

A clash in thinking over regulatory alignment between the UK and the EU is already shaping up to be a central sticking point, with the EU insisting on alignment in exchange for access to the single market and the UK threatening to throw up a customs wall around Britain.

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Varadkar called on the UK to tone down the “nationalist rhetoric” and said negotiations would get off to a better start if they did “not repeat some of the errors that were made in the last two and a half years”.

“Let’s not set such rigid red lines that makes it hard to come to an agreement and let’s tone down the kind of nationalistic rhetoric,” he told the BBC.

He said reports that British ambassadors were being instructed to sit away from EU ambassadors at international events was “silly”.

“I think it just comes across as a little bit petty. It’s kind of when you are in primary school and in secondary school and you get worried about who you can sit beside in class. Most international forums I have attended, whether it’s the UN or other international bodies, you tend to be seated in alphabetic order or protocol.”

He said it was “possible to have a Canada-style trade agreement” but it would not make sense because of the UK’s geography. “Canada isn’t the UK,” he said. “It isn’t geographically part of the European continent. We share seas, we share airspace.”

Critics of Varadkar have claimed he has become much more bullish about the UK’s Brexit because he is facing a difficult general election in Ireland.