A barrister and part-time judge is being investigated for allegedly lying to police about her dealings with the press over Chris Huhne's speeding points-swapping.
Constance Briscoe was arrested after it emerged that she apparently lied when she told police she had not had any contact with the media, jurors at Vicky Pryce's retrial heard.
Pryce, 60, of Clapham, south London, denies perverting the course of justice by taking speeding points for Huhne in 2003, claiming marital coercion.
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The former energy secretary is facing jail after pleading guilty to the offence and will be sentenced after the conclusion of Pryce's retrial, which started on Monday at Southwark Crown Court.
The court heard that Ms Briscoe, a friend and neighbour of Pryce, knew of the points-swapping in 2003.
Jurors were told that Pryce, helped by Briscoe, leaked the story to the Mail on Sunday to get revenge on Huhne for leaving her for PR adviser Carina Trimingham the previous year.
But Ms Briscoe has not been called as a witness in Pryce's trial as she is under police investigation and could not be relied on as a "witness of truth".
Essex Police Detective Inspector Martin Passmore said: "Ms Briscoe has provided statements to the police in this case but during the investigation it became apparent that she may have lied about her involvement with the press.
"For this reason Ms Briscoe has been arrested and is currently under investigation by the police."