The job cuts at the Cowley plant, near Oxford in Britain's southwest, follow a review of operations at the factory, which will also see production downscaled from seven days a week to five.
BMW said while it had "been weathering the economic downturn, it is not immune from the challenges of the current situation".
"Against this backdrop the company felt that a review of its shift patterns was necessary. This decision has not been taken lightly.
"The plant's union representatives have, of course, been involved in the discussions," it said in a statement.
Sales of the Mini slumped 34.5 per cent in January to just over 10,100 worldwide, a spokeswoman told the AFP news agency.
Derek Simpson, Unite's other joint leader, said the job losses placed "a huge onus on the government to take drastic action to support the motor industry and to encourage people to buy cars''.
"The banks will also have to start making credit available again or this is going to lead to disaster," Simpson said.
A spokesman for Gordon Brown, the British prime minister, said that the government was working on a package of relief measures.
Michael Ellam said: "Clearly this is very disturbing news and all I can say is that the government is doing and will do everything it can to help those affected.''