Police last night were investigating claims Tommy Sheridan has been bugged after a suspicious device was found in his car.

The Solidarity MSP yesterday handed the "bug" over to police at the Scottish Parliament after it was uncovered by one of his assistants.

The Herald understands that detectives are taking the investigation seriously and that the device is not of the kind used by British security services.

The discovery is the latest high-profile twist in Mr Sheridan's roller-coaster career and comes after he won his libel action against the News of the World last summer.

Mr Sheridan last night said: "I reported an incident to the police early this afternoon and police are now investigating that."

Police have pored over his car, a silver Honda Civic, and are now examining the device found by Mr Sheridan's assistant.

Sources last night said they did not know why the Glasgow MSP chose to make his report at the Scottish Parliament rather than at his local police office. It is understood Mr Sheridan's assistant, a researcher, "stumbled across" the device in the car but police yesterday declined to say exactly where it was found.

Yesterday evening officers were seen accompanying Mr Sheridan into the office he shares with fellow Solidarity MSP Rosemary Byrne at the Scottish Parliament.

Chief Inspector Brian Stuart, of Lothian and Borders Police, said: "We've received a report in relation to an MSP. A device was found and we are making inquiries into the matter."

Mr Sheridan has previously warned of a surveillance culture in Britain, citing claims senior National Union of Mineworkers officials were bugged during the miners' strike.

Many of his supporters share his view that leftist groups and politicians are watched and listened to by the security services.

Mr Sheridan has even suggested sinister forces from the world of espionage may have been trying to discredit him.

He won his libel victory last August after the News of the World printed lurid allegations about his private life. The paper has appealed after it was ordered to pay Mr Sheridan £200,000 in damages. The tabloid later published extracts from a video tape it said showed Mr Sheridan admitting he had visited a swingers club in Manchester.

Mr Sheridan told a conference in Dundee last October: "The state has a fine history of trying to undermine socialist movements or trade union struggles.

"And when the history of this whole episode is written about, I think you'll find that MI5 certainly was involved."

Mr Sheridan walked out of the Scottish Socialist Party, which he had led and co-founded, after 11 SSP members gave evidence against him at his libel trial. He and Rosemary Byrne formed the Solidarity party, and will fight the SSP for the far-left vote in May's Scottish Parliament election.

The Crown Office said last October it had asked police to investigate allegations that witnesses at Mr Sheridan's trial committed perjury.

A spokesman last night would not be drawn on the most recent allegations. He said: "The area procurator-fiscal for Lothian and Borders is in liaison with the police. It would be inappropriate to comment further at this time."

Colin Fox, the MSP who replaced Mr Sheridan as SSP leader, yesterday said he was "flabbergasted" at the latest development.

The Lothians MSP said: "It's another bizarre twist in the tale. I'm as surprised as anybody by this. I don't know who would want to bug Tommy Sheridan.

"I await with interest to find out what this item is that Lothian and Borders Police have picked up."