Theresa May has been warned she will further damage the Brexit process if she appoints a “patsy” as her new envoy to Brussels.

Former top diplomats raised the alarm after right-wing newspapers were briefed that a wholehearted Brexit supporter will succeed Sir Ivan Rogers, who resigned yesterday.

One said the Prime Minister must appoint a successor for “what they know”, not what they believe – while a second went further, warning a “patsy” would bring “disaster”.

2016: The year of Brexit

Foreign Secretary Boris Johnson is said to be among top ministers demanding a replacement who “believes in Brexit” – after Downing Street lost confidence in the outgoing EU ambassador.

Sir Ivan fell out with No.10 and was fiercely criticised by hardline Brexit-supporters after his memo, warning that a new EU trade deal could take up to a decade to achieve, was leaked last month.

But Sir Simon Fraser, the former head of the diplomatic service, said: “When you appoint ambassadors, you don’t appoint them for what they believe – you appoint them for what they know.”

Speaking to BBC Radio 4's Today programme, he added: “We have someone leading this negotiation who believes in Brexit and that is David Davis [the Brexit Secretary] - that is the role of the minister

“The role of the ambassador and the civil servants is to give clear, dispassionate and objective advice.”

And Jonathan Powell, Tony Blair’s former chief of staff, said: “If they want to have someone who is a patsy, who agrees with them, then what is the point of an independent civil service?

“If you are not prepared to have the argument, if you are not prepared to have someone who will tell you what the problems are, then you are going to end up in a disaster.

“And I’m afraid that is what is going to happen with these negotiations if you really go for a patsy.”

But Iain Duncan Smith, the former Cabinet minister, laid bare Eurosceptic anger with Sir Ivan by claiming the Government could not trust him.

He told the same programme that Sir Ivan probably leaked both his own resignation letter and last month’s warning that a post-Brexit trade deal could take a decade to deliver.

Mr Duncan Smith said: “It gets to a point when a civil servant starts to go public on stuff that you, as ministers, can no longer trust that individual.

“You must have absolute trust and cooperation and you cannot have this stuff coming out publicly. This is now the second time.

“It may actually prove that ministers may well be right to say that they weren’t prepared perhaps to trust him in quite the way they would have done with others.”

The clash came after a Government source told the Daily Telegraph that Ms May intends to appoint a new with “the same attitude” towards Brexit – after Sir Ivan was branded a “gloomy pessimist”.

The replacement is expected to be a senior civil servant, but it left open the possibility of the choice being a figure without any previous experience of Whitehall.

Yesterday, the Government said Sir Ivan had merely retired a “few months early”, although his tenure was due to run until nearly the end of 2017.