Socialist politician Tommy Sheridan said he had been told by police the alleged surveillance device discovered in his car was "viable".
The leader of Solidarity told a news conference that the device, which could transmit audio, would be sent for further tests.
Mr Sheridan said a letter he received, tipping him off about the device, would also be sent for DNA-testing.
He said he had wanted to show the letter but was told not to by police.
The MSP said he had received the letter on Wednesday.
Further investigation
He said: "It was full of information which made it very authentic in terms of my life and my movements over the last few months. Or at least the movements of my car.
"However, the police have requested that I do not release this letter because they fear it could be important in any future investigation of a criminal character."
Mr Sheridan said the police investigation was continuing although he had been allowed his silver Honda civic car back.
He said he was "concerned and angry" by the development but he repeated his belief that state security services were not responsible.
The MSP won a high-profile libel victory over the News of the World last year after it printed allegations about his sex life.
Prosecution authorities later instructed police to investigate allegations of perjury by witnesses during the trial.
The newspaper is seeking to overturn the verdict after being ordered to pay Mr Sheridan £200,000 damages and its appeal is set to be heard in December.
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