Honda is reducing the amount of shifts at its Swindon factory from three to two due to reduced demand across Europe.
Honda builds the Civic, Jazz and CR-V at the Swindon plant for the UK and markets in Europe.
A spokesman for Honda told the BBC that the Japanese brand had not seen the growth it had anticipated over the last 12 months.
The shift reduction could threaten up to 340 jobs of the total 3,000 staff working at the factory.
Honda has seen its market share in the UK reduce since the recession in 2007 with registrations falling from 106,018 units and a 4.41% share in 2007 to 55,660 units and 2.46% share in 2013.
Tony Murphy, Unite national officer for car manufacturing, said: “There is no getting away from it, these job losses are a devastating blow, not just for these workers but for the thousands more across the industry whose work is dependent on the Honda plant.
"Today's losses are also a wake-up call to the UK government. The economy is far too fragile to proclaim a recovery - those workers losing their jobs today will find claims that the country is turning a corner an insult. The truth is that there is simply no pick up in the incomes of Honda's customers, either here or in the eurozone. People are not confident and do not have the cash to spend. That is something which must give the Government serious cause for concern.
"Decent, skilled jobs are being lost today and investment is being withdrawn. We cannot allow workers of this expertise to be dispatched to the dole queue - the country needs their skills if we are to power our country back to economic health."