Officers were called out to reports of a “suspicious” vehicle and found the Labour peer’s Mini parked at an angle with the engine and lights still on but no one at the wheel.
They called Dame Joan, 86, who said she was away for the weekend and admitted this was not the first time she had accidentally left the car running because it was a keyless vehicle.
She gave the officers permission to smash a window and disable the locked car. However, not wanting to damage the vehicle, they hatched a plan to stall it using a police evidence bag.
When that failed they decided to leave the car because it was almost out of fuel and was not deemed a safety risk, Willesden magistrates court was told.
Details of the bizarre incident, in Primrose Hill on November 29 last year, were revealed in court papers obtained by the Evening Standard.
Pc James Lewis said he and a colleague, Pc Daultry, were called out at 6.45pm to the road where Dame Joan lives after “a report of a vehicle that was suspicious”.
“It was parked on the street at an angle and the engine was running with the lights left on,” he wrote in a witness statement.
They identified the veteran BBC broadcaster as the registered owner and confirmed over the phone that “the vehicle had not been stolen and she had probably left the vehicle in this position by accident.
“Bakewell told me that the vehicle is keyless and she had left the vehicle on the roadside with the engine still running before. Bakewell gave me permission to break the window in order to switch off the car if required.”
Pc Lewis said they then “made an attempt to stall the vehicle by covering the exhaust with a police evidence bag”, but added: “This did not work.” The officers left “the vehicle in the position it was found in as it was deemed safe and there was only one bar of fuel left”.
Dame Joan was told that evening she might be prosecuted, and she was given the formal police caution of her rights. Pc Lewis said he explained to her that he understood it was “an accident” but “it had a detrimental effect on the local environment and was causing annoyance/alarm to the local community”.
“Lady Bakewell was not able to return to switch off the engine as she was away for the weekend,” he added.
The Met Police brought a prosecution against Dame Joan for leaving a vehicle on a road not attended by a person licensed to drive it and with the engine running.
Her case was due to be heard in March but was delayed by the pandemic and finally dealt with at the end of May, when police withdrew the charge and admitted Dame Joan should never have been prosecuted.
“There appears to have been an admin error and the fine was paid,” the court was told. Court papers did not reveal the level of fine paid by Dame Joan.