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Chuka Umunna has written to Boris Johnson demanding that the Prime Minister answer questions about claims made by a No 10 source that they are investigating MPs for "foreign collusion".

In a letter to the PM, the Liberal Democrat Foreign Affairs spokesman asked Mr Johnson to confirm if he or any officials or ministers had "sanctioned intelligence gathering, surveillance or any other investigation" by the government into contacts "between UK Parliamentarians and foreign governments".

He also asks Mr Johnson to spell out the legal basis for any investigation and if he supports reported bids by backbench Tories to make contact between MPs and foreign governments illegal.

In his letter Mr Umunna writes that it is "vital for backbenchers" to have conversations with governments in order to "inform our work, free from interference from HM Government".

Earlier this month senior No 10 source said told the Mail on Sunday that the government was working on "extensive investigations into Dominic Grieve, Oliver Letwin and Hilary Benn and their involvement with foreign powers and the funding of their activities".

They added that the government "will demand the disclosure of all details of their personal communications with other states".

Describing the bid Mr Umunna said: "It represents an attempt to fundamentally breach of our constitution and our human rights – it would threaten our democracy."

It comes after the Mirror revealed that senior diplomats accused the government of “putting lives at risk” over shadowy briefings accusing opposition MPs of “colluding with foreign powers” for criticising Boris Johnson ’s Brexit strategy.

(Image: UK PARLIAMENT/AFP via Getty Imag)

Two senior figures representing Britain abroad have told the Mirror they will launch complaints this week over briefings from inside No 10 which suggested the government might attempt to seize MPs email and phone records.

Tory MPs announced they would introduce a new law that would see any British citizen who undermines Government negotiations abroad face jail in the wake of Boris Johnson's humiliation.

But the accusation - which all three MPs strenuously deny - sent shockwaves through Britain's diplomatic networks.

(Image: WILL OLIVER/EPA-EFE/REX)

One official told the Mirror: “This is a witchhunt and it risks undermining the work we do abroad telling foreign governments not to prosecute, jail and harass opposition politicians who disagree with them.

"It cuts against more than 50 years of British foreign policy and it will put lives at risk.”

A second official confirmed that staff in the Foreign and Commonwealth Office were incandescent over anonymous briefings from No 10.

“Threatening MPs with investigation is something you would expect the government to be stopping abroad, not encouraging at home, ” they said.

(Image: Adam Gerrard / Daily Mirror)

Senior figures within the diplomatic service are expected to raise behind the scenes complaints with No10 this week - with formal complaints threatened if the rhetoric doesn't die down.

Liberal Democrat foreign affairs spokesman Chuka Umunna said: “In order to ensure Parliament can scrutinise HM Government’s foreign policy and hold Ministers to account for their actions, it is vital for MPs from opposition parties to have dialogue with other governments. It informs our work for the British public, free from interference by HM Government.

“If Boris Johnson’s Government has sanctioned an intelligence probe into these links, it would represent an attack on our constitution and our democracy. Not only is the legality questionable but the mere threat of this action amounts to the harassment and intimidation of MPs which one associates with authoritarian regimes, not mature democracies.”