The spokeswoman said May, who has called a snap election for June 8, had invited Juncker to discuss the process of withdrawal. Three days later on April 29, the other 27 EU leaders will meet in Brussels to agree their negotiating positions. The EU expects to start talks after the British election.

Asked about reports that May will pledge ahead of the June 8 election to end the free movement of EU citizens into Britain, the spokeswoman reaffirmed the EU's stance that people's rights would be respected and should not be used as bargaining chips.

"We want to reassure EU citizens living in the UK and UK citizens living abroad in the EU that their rights will remain respected and enforceable in a non-discriminatory way so this will be a very prominent part in the negotiations once they formally start," said Mina Andreeva.

Asked about a Financial Times report on an memo to Commission managers from Juncker aides telling them not to issue contracts to British-based firms if that would conflict with rules on hiring only EU-based contractors after Brexit, she said Britain would be treated equally until it leaves, in 2019.

"The Commission will ... respect the rights that the UK enjoys as a result of its continued membership," Andreeva said. "And everything that is after we will see after."

EU officials, however, said that it was "only natural" that the Brussels executive starts anticipating the impact of Brexit, now that May has formally triggered the legal withdrawal period.

Diplomats say that their British counterparts already appear increasingly sidelined in discussions in Brussels and the trend will pick up pace as the Union exercises its right to organise legislative meetings of the 27 other states without Britain.

Leaders of the 27 will meet for the first time as a formal lawmaking body when they hold their European Council on April 29 to agree the guidelines by which the Commission's Brexit negotiator, Michel Barnier, will steer the talks with London.