Theresa May must take new UK ambassador to EU into her confidence for Brexit to succeed, warns ex-diplomat

Sir Robert Cooper said a difficult Whitehall atmosphere is due to officials working in a ‘policy vacuum’

The Prime Minister has promised to set out her negotiating strategy for leaving the EU this month (Getty)

Theresa May must take the UK’s new ambassador to the EU into her confidence if Brexit talks are to succeed, an ex-British diplomat has warned.

Sir Robert Cooper said the relationship between the Prime Minister and newly-appointed Sir Tim Barrow would be “critical” to the chances of securing a good post-Brexit settlement.

He argued the atmosphere in Whitehall is strained because Downing Street is currently asking officials to work in a “policy vacuum”, giving them no clear direction.

It comes amid concerns that Ms May’s administration is operating among an increasingly small inner circle of trusted aides, while Sir Tim’s predecessor Sir Ivan Rogers was said to have quit his post after losing the Prime Minister’s confidence.

Sir Robert, now an adviser to the EU Commission, said: “[Sir Tim’s] relationship, his personal confidence with the Prime Minister, will be absolutely critical.”

He explained that there has to be “one team” for the UK negotiations, involving “all the people in Brussels and all the people in London”.

Sir Robert told BBC Radio 4’s Today programme: “You always spend at least as much time negotiating with your own side as with the other side, the important thing is to go into the negotiations with a position that takes account of what others are going to do.

“It’s no good people in Whitehall negotiating with themselves – there are 27 others out there, that’s the real negotiation.

“So it’s very important that Tim should be closely involved in all the discussions going on in London.”

At the end of last year Ms May’s office was forced to state that she worked in an “inclusive” manner, following claims from top officials that she is centralising power more than her predecessor David Cameron.

Meanwhile Tory MPs also today defended the need for civil servants to be listened to when delivering uncomfortable news to ministers.

Discussing the situation Sir Tim will now have to deal with, Sir Robert added: “At the moment there is a policy vacuum. It’s not surprising. This is a gigantic enterprise that’s been taken on, it needs a lot of thought.

“At the moment the atmosphere is difficult because people don’t know where they are going. You need a sense of direction.”

Sir Ivan resigned while delivering a veiled attack on Britain's Brexit negotiating strategy, urging his colleagues in Brussels to challenge “muddled thinking and ... speak truth to power”.

Ms May is due to make a speech later this month in which she has promised to set out further detail of her negotiating strategy.