Theresa May has accused European politicians and officials of seeking to influence the General Election by making threats to the UK.
In a speech outside Downing Street, the Prime Minister - who has been accused by Labour of stoking a confrontation for appearance's sake - said the European Commission had hardened its position to coincide with the 8 June poll.
"The events of the last few days have shown that whatever our wishes and however reasonable the positions of Europe's other leaders, there are some in Brussels who do not want these talks to succeed, who do not want Britain to prosper," she said.
Speaking after meeting the Queen to mark the dissolution of Parliament, Mrs May said the winner of the election would face "one overriding task" - getting the "best possible deal" for the UK.
"In the last few days we have seen just how tough these talks are likely to be," she said.
"Britain's negotiating position in Europe has been misrepresented in the continental press. 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 that will take place on 8 June."
Reports have emerged Britain could be asked to pay as much as €100bn (£84.5bn) as part of its divorce settlement with the EU.
Meanwhile, the EU's chief Brexit negotiator has said talks on Britain's exit will not be concluded "quickly and painlessly".
And at the weekend there were claims a Brexit dinner involving Mrs May and European Commission President Jean-Claude Juncker was a "disaster".
In her address outside Number 10, Mrs May said the election was not only about Brexit, but also about building a stronger and fairer economy, protecting the UK against attacks from extremists and efforts by separatists to tear it apart.
But Europe was the overriding theme, with nine mentions of the word "Brexit" and no mentions of "Conservative", "Conservatives", "Tory" or "Tories".
The PM warned of "serious consequences" if the Brexit talks went wrong, saying it was "central to everything".
She said: "If we do not get this right, the consequences will be serious and they will be felt by ordinary working people across the country.
"This Brexit negotiation is central to everything.
"If we don't get the negotiation right, your economic security and prosperity will be put at risk and the opportunities you seek for your families will simply not happen."
The PM said she had made clear to Brussels in her letter formally starting Brexit that "in leaving the European Union, Britain means no harm to our friends and allies on the continent".
"We continue to believe that no deal for Britain is better than a bad deal, but we want a deal," she said.
"We want a deep and special partnership with the EU and we want the EU to succeed."
Labour leader Jeremy Corbyn accused Mrs May of "playing party games" in the "hope of winning advantage" for the Tories.
He said: "By winding up the public confrontation with Brussels, the Prime Minister wants to wrap the Conservative party in the Union Jack and distract attention from her Government's economic failure and run down of our public services.
"But Brexit is too important to be used as a political game in this election.
"These are vital negotiations for every person in Britain and for the future of our country. But Theresa May is putting party interest ahead of the national interest."
The party focused its campaign on health on Wednesday, saying a Labour government would halt NHS plans intended to better co-ordinate local services because of concern they are a vehicle for cuts, closures and privatisation.