CAR company Nissan is investigating after 20 cars were damaged in what may have been a protest at overtime.

Production line workers at the plant, in Washington, Wearside, said the cars were attacked, causing up to £200,000 damage, during a night shift, and three had been written off.

It is thought the protest may have been triggered when bosses added an extra hour of "short-notice overtime" to the end of the shift.

A spokesman for Nissan, which employs nearly 5,000 people at the plant, confirmed yesterday that the company was investigating, but could not confirm whether any of the 20 cars had been written off.

"As to how the damage occurred, we are still looking at that," he said. "We build more than 1,500 cars a day, so for cars to get damaged while they are in production is not unheard of."

The incident is believed to have happened on Monday, on Line One at the plant.

Line One produces the Almera and Primera, and started work on the Note - the latest model to enter production at Sunderland - last month.

A source said: "It was done by lads who are sick of the extra overtime they have had to do.

"The company has been putting short-notice overtime on top of the late shift hours. They just stick it on the end of the shift and you have to work it - there isn't any choice."

He added: "We saw the damaged cars coming down the line."

Amicus regional officer Dave Telford said he had not been informed of any incident, but the fact that Nissan had launched an investigation made him believe something must have occurred.

"It is not unusual for them to experience damage to cars - but there would not usually be an investigation," he said.

"I have had members contact me to complain about short-notice overtime, particularly on the late shift, but Nissan are within the European Working Hours Directive and such overtime is contractual. There is not a lot you can do about it."

Sunderland is the most productive car plant in Europe, and has been for the past seven years.

It is also the UK's biggest car plant, producing one in every five cars built in the UK during 2003.

At peak production, 90 cars an hour roll off its twin lines and the production process takes just over 15 hours.