The government’s adviser on political violence has written to the home secretary asking to investigate the intimidation of candidates during the general election.

Lord Walney is suggesting there could have been a “concerted campaign by extremists”.

He is urging Yvette Cooper and Security Minister Dan Jarvis to commission a short inquiry to find out if groups in different constituencies were working together and to document what he calls the “dark underbelly” of abuse.

In the letter, seen by the BBC, Lord Walney said evidence from the last couple of months points to a “concerted campaign by extremists to create a hostile atmosphere for MPs within their constituencies to compel them to cave into political demands”.

He writes the “conduct of the election campaign in many communities has underlined the gravity of the threat to our democracy” from the abuse and intimidation of politicians, local and national.

Lord Walney said: "I am increasingly concerned about the scale of intimidation against candidates in the general election.

“I believe there is now a need for a focused piece of work on the scale and drivers of this intimidation so that it cannot continue to mar our democratic processes and put candidates at risk.”

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His concerns follow comments from the Speaker Sir Lindsay Hoyle, who told the BBC: “if there is something that keeps me awake at night, it is the safety of MPs”.

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Sir Lindsay said he had “never seen anything as bad” as the current level of intimidation.