The former aide of a Labour MP accused of lying to avoid a speeding ticket said he felt "morally and legally" obliged to give evidence against her.
Dr Christian DeFeo said he contacted police after seeing a news report of Peterborough MP Fiona Onasanya's trial at the Old Bailey.
Fiona Onasanya claimed a Russian man was behind the wheel when her car was clocked doing 41mph in a 30mph zone.
The 35-year-old denies one count of perverting the course of justice.
The prosecution has claimed Ms Onasanya and her brother Festus Onasanya "acted jointly in telling lies" to claim she was not driving her Nissan Micra when it was clocked by a speed camera in July 2017, a week after she was elected as an MP.
Dr DeFeo told the court he had "enormous hopes invested" in the Labour MP, continuing: "I never dreamed in my darkest dreams - I never thought I would have to be sitting here [giving evidence].
"It is with the greatest reluctance I have to do this.
"To do otherwise, I cannot. It's morally and legally unacceptable not to."
The MP's former campaigns and communications manager said she visited his house on 24 July and spent no less than two hours discussing the "nitty gritty" of a charity lease.
He said he could not give a specific time but said it had been "quite late" and she had arrived in her car alone.
The court has heard Ms Onasanya was sent a Notice of Intended Prosecution (NIP) after her car was clocked in Thorney, near Peterborough, at 22:03 BST on 24 July.
She returned the paperwork naming the driver as Aleks Antipow, a previous tenant of a property the siblings rented out in Cambridge.
The jury heard he was in Russia at the time of the incident had had never driven the MP's car.
On 5 November, Mr Onasanya, 33, of Chesterton, Cambridge, pleaded guilty at the Old Bailey to three counts of perverting the course of justice.
The trial continues.
- 13 November 2018
- 5 November 2018
- 13 August 2018
- 27 July 2018
- 25 July 2018