Honda has been based at Swindon for more than 10 years

Amicus said hundreds of workers at the Swindon factory will be affected, with some losing shift allowances worth up to 17% of their pay.

The union said it believed between 150 and 200 temporary jobs would not be renewed under the plans.

Joint general secretary Derek Simpson said: "This is come like a bolt out of the blue and has come in the middle of pay negotiations.

'Cynical ploy'

"We hope this is not a cynical ploy to influence the outcome of those negotiations."

Honda confirmed it was making changes to shifts which it said would affect around a fifth of its 4,300-strong workforce.

The company said it was trying to make a profit this year following cumulative losses of �400m over the past 10 years.

A company spokesman said Honda was looking to produce record numbers of cars this year at Swindon with a target of building 190,000 vehicles.

The spokesman stressed there would be no redundancies and said no final decision had been made about the position of temporary staff.

"We feel we can make efficiency savings," said the spokesman.