British Prime Minister Theresa May on Wednesday accused European Union officials of trying to influence the UK elections, ratcheting up tensions with Brussels over her country's departure from the bloc.

Just hours after the EU unveiled its plan for Brexit talks, which delays discussion on vital issues such as trade, May said that "the European Commission's negotiating stance has hardened. Threats against Britain have been issued by European politicians and officials".

"All of these acts have been deliberately timed to affect the result of the general election," she told reporters outside 10 Downing Street, in a speech aimed at rallying support for her Conservative Party ahead of the snap general election to be held on June 8.

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"I think it's a massive escalation and it's truly extraordinary language," said Al Jazeera's Paul Brennan, reporting from London.

Brennan said May had offered no specific proof or examples of how the EU had allegedly interfered in the UK's democratic election process.

"It's almost pressing the nuclear button on the Brexit talks and it certainly doesn't bode well for an accommodating spirit as the talks actually get started in the middle of June," said Brennan.

Brexit talks

May's comments follow leaked comments in the British and European press suggesting the EU thinks Britain is not being realistic about the conditions of its EU exit and the complexity of the negotiations ahead.

A German news report said that Jean-Claude Juncker, head of the EU's executive commission, left a private dinner with May last week saying he was "10 times more sceptical than I was before" that negotiations will succeed. May dismissed the report as "Brussels gossip".

Michel Barnier, the EU's chief Brexit negotiator, earlier on Wednesday insisted that Britain's accounts must be settled before any talks on its future trade relationship with the bloc can take place, as he warned that time is running out to seal a deal by the 2019 deadline.

Unveiling the commission's negotiating mandate for Britain's departure, Barnier said he was not hostile to Britain and the EU did not want to punish it for leaving, but "we have to settle the account, not more not less".

Barnier did not specify how much Britain should pay, but his negotiating mandate said it should cover budget payments, the cost of ending Britain's membership of any EU institution including banks, and the bill for relocating any EU agencies on its territory. London must also pay in euros rather than pounds, meaning that it should bear the currency exchange costs.

Under the negotiating mandate, EU states would have to approve progress on these and other immediate exit issues before Barnier can start negotiating the outline of the bloc's future relations with Britain after it leaves.

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The amount London owes the EU has become one of the most contentious subjects in the Brexit process, with some reports estimating it could climb to as much as 100bn euro ($109bn) - a figure that Britain has flatly refused to pay.

Money aside, the top priority of the talks is how to handle the rights of some three million citizens from the 27 nations living in Britain and up to two million Britons residing on the continent. All face massive uncertainty on such issues as health benefits, pensions, taxes, employment and education.

Another key aim is to keep people and goods moving smoothly across the border between Northern Ireland, which is part of the UK, and the Republic of Ireland, which is an EU member.

Some British politicians, including May, have said walking away would be better than a bad deal, and Barnier conceded that the EU has planned for such a contingency.

"We are prepared for all options," he said. "But the option I am working on is getting an agreement."

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Al Jazeera's Brennan said the EU's stance shows they envisage a "double-phase" Brexit, and that the settlement estimate had ramped up the pressure on May.

"To some people within Theresa May's supporters, they don't want to pay the European Union anything," said Brennan. "So to be told that they might have to pay $110bn upfront, you can see that really didn't go down very well."

This is the first time a member has ever left the EU, so these negotiations are unprecedented and complicated, and combined with fresh delays caused by the snap elections in Britain, are raising tensions between London and the 27 nations that will remain in the bloc.

Before making her allegations about the EU, May signalled the official start of Britain's Brexit-dominated election campaign on Wednesday with a visit to Queen Elizabeth II.

May called on the Queen at Buckingham Palace to mark Wednesday's dissolution of Parliament, which means Britain no longer has any elected politicians, though the government continues to function. All 650 seats in the House of Commons are up for grabs in the June 8 election.