Anti-capitalism activists wearing masks of Britain's Prime Minister Theresa May (L) and US President Donald Trump | Odd Andersen/AFP via Getty Images Brexit Files Insight Fear of rogue Britain What if the UK goes full Donald Trump after a Brexit extension?

Jacob Rees-Mogg gave voice to the very thing EU ambassadors had been worrying about.

In a tweet sent on Friday, the leading Conservative Brexiteer said that if British Prime Minister Theresa May is forced to accept a long extension of Article 50, the U.K. should be “as difficult as possible” to punish Brussels.

Rees-Mogg said Britain could veto budget proposals and obstruct Emmanuel Macron’s reform plans.

In Brussels, this was seen as no empty threat. In fact, it was exactly that prospect that had been bothering EU ambassadors.

What if the U.K. goes full Donald Trump after it is allowed to stay? What if it really started causing trouble? The nice Mrs. May might not, but Boris Johnson could.

The ambassador of one big EU27 member country told POLITICO: “The most important question now is how is this going to affect the EU. We want to have a very sensible solution for the U.K., but our main fear is how it will affect the EU27.”

If Brexit is delayed for several months — rather than until June 30 as May is requesting — it opens up a whole raft of political possibilities currently not available: from a general election and a referendum, to a new hard-line Tory leader pursuing no-deal.

“Are we going to be able to work efficiently in the coming months with a country that is unwilling to agree an agreement that is already signed?” the ambassador asked, speaking on condition of anonymity.

It’s not just a question of irritation either — could a long extension give a Brexiteer prime minister new leverage in the Brexit negotiations by preserving the U.K.’s ability to upset the workings of the EU until it got its way?

“Would it have new leverage that would serve as an instrument to reopen the Withdrawal Agreement? Can we put parts or all of the machinery of the EU in the hands of the U.K.? The EU’s functioning has to be taken into account,” said the ambassador.

Mindful of the prospect of the U.K. going rogue, a draft seen by POLITICO of the European Council conclusions that EU leaders are due to agree later this evening includes a clause demanding “constructive and responsible” cooperation from the U.K. during a long extension. It mandates that the U.K. “shall facilitate the achievement of the (EU’s) tasks and refrain from any measure which could jeopardise the attainment of the (EU’s) objectives.”

The U.K. has form for using such tactics.

In the late 1990s, yet another Tory prime minister hamstrung by his party’s divisions over Europe — John Major — launched a sabotage strategy to force the EU to lift its ban on British beef following the mad cow disease crisis.

Major ordered his ministers to block reforms in Brussels — even ones championed by the British — in a bid to force the EU back to the negotiating table.

Britain had already won a reputation for being “obstructionist and negative” through its then Europe minister — one David Davis — who earned the nickname “Mr. No” for his uncompromising diplomacy.

The problem: The strategy failed. The ban on British beef long outlasted Major and Davis’ time in office.

The worldwide ban on British beef exports was only lifted by the EU in 2006 — 10 years after it was first imposed. Labour’s Tony Blair had won three general elections in the meantime.

“The problem was it wasn’t really that effective,” said one veteran Tory MP involved. “I don’t think the EU should worry too much to be honest.”

Indeed, the only time a British prime minister yielded the U.K. veto in the European Council proved equally useless. In 2011 David Cameron vetoed an EU treaty change to tackle the eurozone crisis because he could not negotiate opt-outs for the City of London. In the end, the rest of the EU found a mechanism to do what they wanted anyway — without the U.K.

This article has been updated to amend the year in which David Cameron vetoed an EU treaty change

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