An aide to Peterborough's MP has said he feels "morally and legally" obliged to give evidence against her in court.
Fiona Onasanya is on trial accused of plotting with her brother to avoid a speeding ticket.
The court was told of an email exchange between her and her former campaigns and communications manager Dr Christian DeFeo.
In it they agreed to meet at his house, close to where the incident happened, on the day the speeding ticket was issued.
Onasanya is accused of planning with her brother Festus Onasanya to claim that a Russian man was behind the wheel of her Nissan Micra when it was clocked doing 41mph in a 30mph zone on The Causeway near Thornley in Cambridgeshire.
Onasanya 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 July 24 incident.