Britain’s civil servants are struggling to meet Theresa May’s tight timetable for Brexit and feel they do not have “clear political direction”, the former head of the Foreign Office warned on Wednesday.

Britain will need more civil servants after its leaves the EU, said Sir Simon Fraser, arguing that stripping away a layer of bureaucracy in Brussels could simply lead to more red tape at home.

His comments increased pressure on the prime minister to give greater clarity about her aims in Brexit negotiations, but she stuck rigidly to her defensive lines at a rowdy session of prime minister’s questions.

Mrs May’s silence on her key Brexit demands has created a vacuum, with senior Whitehall officials growing increasingly gloomy about the prospects of a harmonious deal with the EU.

Some officials have told the FT that they are preparing to quit, saying that in spite of their efforts the prime minister seemed intent on leading the country towards a “hard Brexit”. “I’ve done my best,” said one.

Sir Simon, former permanent secretary of the Foreign Office, gave voice to his former colleagues’ frustrations when he said: “Civil servants don’t feel they have clear political direction at the moment.”

He told the Commons Brexit committee that officials were working “incredibly hard” but that Mrs May’s plan to start exit negotiations in March 2017 was looking ambitious. “The afterburners need to be put on,” he said.

A leaked report by a Deloitte consultant suggested that up to 30,000 civil servants might be needed to deal with Brexit, and Sir Simon agreed extra staff would be essential after Britain leaves the EU.

To the consternation of pro-Brexit Tory MPs, he said border guards, customs officials and trade experts would be among staff Britain would have to recruit once exit was complete.

Sir Simon said the forthcoming UK-EU trade negotiation would be unlike regular deals in the sense the outcome was “likely to be a reduction in trade” because of new tariffs and other barriers.

The 30,000 figure, furiously denied by Number 10, is roughly equivalent to the entire European Commission staff in Brussels. It far exceeds the extra 500 being hired by the government to staff new Brexit ministries.

But hiring new customs officers alone after Brexit could increase the civil service headcount significantly: the UK has 5,000 officers dealing with the EU’s external tariff wall, compared with 35,000 in Germany and 16,000 in France.

This week Boris Johnson, foreign secretary, told a Czech newspaper that Britain would probably leave the customs union, while Mrs May told MPs cryptically that membership was “not a binary choice”.

Turkey is an example of a non-EU country where some sectors, such as manufacturing, are part of the customs union. Downing Street said Mrs May was not necessarily talking about a sector-by-sector approach.

While some younger civil servants are grasping the opportunity to work on the biggest Whitehall challenge for a generation, longer-serving staff are becoming disillusioned with what they see as the government’s unrealistic approach to Brexit.

That breezy optimism was typified by Mr Johnson’s assertion in September that “our policy is having our cake and eating it. We are pro-secco but by no means anti-pasto.”

Mr Johnson’s approach was described on Wednesday as “insulting” by Carlo Calenda, a former Italian envoy to Brussels, who told Bloomberg Television that Britain was in a state of “chaos” on Brexit.

He said Mr Johnson told him that Italy would sell less prosecco if the EU barred Britain from the single market. “He basically said, ‘I don’t want free movement of people but I want the single market’,” Mr Calenda said.

“I said, ‘no way’. He said, ‘you’ll sell less prosecco’. I said, ‘OK, you’ll sell less fish and chips, but I’ll sell less prosecco to one country and you’ll sell less to 27 countries’.’’ Mr Calenda added: “Putting things on this level is a bit insulting.”

Jeroen Dijsselbloem, the Dutch finance minister, told BBC Television’s Newsnightthat Mr Johnson was “saying things that are intellectually impossible, politically unavailable”.

An Ipsos MORI survey found 48 per cent of the public thought the government was doing a “bad job” at handling Brexit compared with 37 per cent who thought it was doing a good job.

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