The European Union’s Brexit negotiators fear that they are being bugged by the British secret service after the UK obtained sensitive documents “within hours” of them being presented to a meeting of EU officials last month, The Telegraph understands.

A highly placed EU source revealed the security concerns as British negotiators were set to return to Brussels on Thursday to resume Brexit talks.

The two sides remain far apart on the key issues of customs arrangements and Ireland, with Latvia’s foreign minister warning on Wednesday that the risk of a ‘no deal’ outcome was now “50-50”.

The EU bugging fears were raised at a meeting in the European Council on July 13 by a top member of Michel Barnier’s Brexit negotiating team, according to the source.

Sabine Weyand told EU officials attending the meeting of the European Council’s Article 50 Working Party that “it could not be excluded” that British intelligence had penetrated their meetings, the source said.

The EU fears were spawned after British negotiators obtained the contents of a politically explosive slide presentation almost immediately after they were shared on July 5 - just a day before Theresa May’s crunch Chequers summit.