The affected cars were built between March 1989 and September 1990. Many of these Fiestas are used by driving examiners, a number of whom have been injured in accidents during driving tests. Some examiners, including one now in constant pain after smashing into the windscreen, are preparing to sue Ford.
Ford said its investigations identified a fault in one of the two sensing mechanisms which allow the belt to lock.
The Driving Standards Authority, which supervises driving tests, had urged Ford to recall the Fiestas after cases of belts not working.
The Labour MP Gwyneth Dunwoody, a member of the Commons transport committee, has campaigned in the case. The MP is also a member of the public services, tax and commerce union the PTC. The union's solicitors, Thompsons, are acting on behalf of seven driving examiners hurt in accidents.
According to an article in last month's Sunday Times, the DSA told Ford as long ago as October 1994 it was concerned about Fiesta seatbelts. The case was also highlighted on BBC1's Watchdog programme.
Ford denied dragging its heels on the problem. "We have been investigating this since it was first brought to our attention. It is only now that we have identified a specific problem that we can act upon."