The biggest settlement is with Honda of Japan, which is alleged to have disabled the system monitoring exhaust emissions on 1.6m of its cars.

The company will pay a fine of more than $17m, but correspondents say the total cost to the company will be swollen by a further $250m for corrective engine checks and repairs.

Honda also agreed to spend an additional $4.5m to implement various environmental projects.

"This settlement means cleaner air for the American people," said the US Attorney General Janet Reno. "It will help eliminate thousands of tons of exhaust emissions.... And it will help save Honda owners money because they will get an early warning that their car needs repair."

The second case involves Ford, which was accused of tampering with emission controls on 60,000 of its Econoline vans to improve fuel efficiency. The settlement will cost Ford nearly $8m, including a fine of $2.5m.

The government alleged that Ford illegally installed a device that defeated the emissions-controls system in the 1997 Econoline vans, increasing fuel economy but causing higher tailpipe emissions at highway speeds.

"This action is another step taken by this administration to ensure that companies fulfill their responsibility to protect public health and environment," said Environmental Protection Agency administrator Carol M. Browner.