European Council President Donald Tusk on Thursday called for "mutual respect", one day after British Prime Minister Theresa May claimed that "bureaucrats" in Brussels were trying to affect the upcoming snap election in Britain and ruin Brexit talks.

"Brexit talks difficult enough. If emotions get out of hand, they'll become impossible. Discretion, moderation and mutual respect needed," Tusk tweeted, alluding to May's remarks.

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"At stake are the daily lives and interests of millions of people on both sides of the Channel," he added.

A spokesperson of the European Commission earlier also brushed aside May's claim, saying the EU was too busy to care about British election.

"This election in the United Kingdom is mainly about Brexit, but we here in Brussels, we are rather busy with our policy work, we have too much to do on our plate," said Margaritis Schinas, the European Commission's chief spokesperson.

"When we have a British government immediately after the UK election, we'll be very busy in engaging constructively in Article 50 negotiation for which we are ready," he added.

May on Wednesday made her most stinging remark so far against EU "bureaucrats" in Brussels, accusing them of trying to ruin Brexit talks.

In her briefing outside 10 Downing Street, May said the events of the last few days had shown that whatever Britain's wishes, there were some people in Brussels who did not want Brexit talks to succeed and did not want Britain to prosper.

"The European Commission's negotiating stance has hardened. Threats against Britain have been issued by European politicians and officials," May said.

The British Prime Minister said these acts had been deliberately timed to affect the result of the upcoming general election.

May and the Conservative Party remain well ahead in the opinion polls, pointing to a victory for May with an increased majority on June 8 if the pollsters are right.