Dr Christian DeFeo contacted police on Tuesday after seeing a news report of the trial of Labour's Fiona Onasanya at the Old Bailey.
He told the court: "It is with the greatest reluctance I have to do this.
"To do otherwise, I cannot. It's morally and legally unacceptable not to."
Onasanya is accused of plotting with Festus Onasanya to claim a Russian man was behind the wheel of her car when it was caught travelling at 41 mph in a 30 mph zone near Thorney in Cambridgeshire on 24th July 2017.
The 35-year-old representative for Peterborough denies one count of perverting the course of justice.
Her brother has pleaded guilty to three charges of perverting the course of justice, including to one related to the 24th July incident.
The speeding incident happened close to where Dr DeFeo lived at the time.
He told the court how Fiona Onasanya arrived at his home on 24th July to discuss a charity lease, adding they were together for at least two hours.
Dr DeFeo added that he could not pinpoint the exact time of her arrival but it had been "quite late" and she arrived in the Nissan Micra on her own.
Dr DeFeo and his wife claim Onasanya arrived just after 10pm - rather than at around 5pm as the defence suggests.
The court heard the speed camera was set off shortly after 10pm.
The court also heard there had been a "bit of a falling out" between the politician and Dr DeFeo's wife after Onasanya cancelled an event marking a century of Labour in Peterborough.
Dr DeFeo denied a suggestion by prosecutor David Jeremy QC that he "had it in" for Onasanya.
He said: "It's a terrible thing to have to do this [give evidence]."
The trial continues.
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