Foreign academics at a leading British university have been barred from offering advice on Brexit, apparently because of “national security”.

A message sent by the Foreign Office informed staff at the London School of Economics (LSE) that their contribution to Government analysis and reports was no longer wanted.

A source told The Independent that the instruction had been given because sensitive trade relations were being examined, adding: “They do not want academics from overseas involved – it seems to be about national security.”

The crackdown was revealed by Sara Hagemann, an assistant professor at the LSE who specialises in EU policymaking and treaties and in the role of national parliaments.

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Dr Hagemann tweeted: “UK govt previously sought work & advice from best experts. Just told I & many colleagues no longer qualify as not UK citizens.”

The Foreign Office is believed to dispute the claim, but had not responded to requests for a response by the time of publication.

The move was condemned by Nick Clegg, the Liberal Democrats’ EU spokesman, who said: “It is utterly baffling that the Government is turning down expert, independent advice on Brexit simply because someone is from another country.

“This is yet more evidence of the Conservatives’ alarming embrace of petty chauvinism over rational policymaking.”

How the pound has struggled since Brexit

The decision comes despite Britain’s well-advertised lack of trade negotiators, which could threaten its ability to carry out detailed and protracted talks with the remaining EU countries.

Dr Hagemann could not be contacted, but the LSE confirmed that an instruction had been received from the Foreign Office, which would affect a number of its foreign academics working on different policy briefs.

A spokesman said: “The UK Government regularly calls upon LSE’s world-class academics for their advice on a range of issues.

“We believe our academics, including non-UK nationals, have hugely valuable expertise, which will be vital in this time of uncertainty around the UK’s relationship with Europe and the rest of the world.”

The spokesman added: “Any changes to security measures are a matter for the UK Government.”

There are suggestions that one of the academics affected is a dual-national, with citizenship of both the UK and another EU member state.

Steve Peers, a professor of EU law at the University of Essex who has advised the Government on EU legal questions, said it should be possible to receive advice without sensitive information being revealed.

“I don’t really get the security or sensitivity argument,” he said. “Whatever the reasons, this will come across as hostile, narrow and xenophobic.”

The move comes amid strong criticism of the Government over immigration – in particular, the proposal to require companies to declare the proportion of international staff in their workforce.

Ministers suggested those employers with the highest proportions of foreign staff could be “named and shamed” for not training and employing British people.

A Foreign Office (FCO) spokesman said: “The FCO regularly works with academic institutions to assist in its policy research and nothing has changed as a result of the referendum.

"It has always been the case that anyone working in the FCO may require security clearance depending on the nature and duration of their work.