The consumer affairs show claims that Ford issued a technical service bulletin advising engineers to replace part of the suspension of Mondeos which veer to the left when braking.
This document advises dealers that if a customer expresses concern about the vehicle pulling to one side during braking, they should consider installing "revised suspension arms".
But Ford, whose cars have been the subject of numerous Watchdog investigations, said there was "no evidence of a design or manufacturing defect with the steering". The company added that it complained to the Broadcasting Complaints Commission about Watchdog's "unfair" handling of the affair.
Watchdog stood by its story. One independent vehicle inspector, Paul Piggott, said on the programme: "It definitely pulls to the left under heavy braking, which it shouldn't do."
Ian Griggs, an engineer from Peterborough, said his Mondeo LX had a problem of pulling to the left, which was exaggerated on braking. "If you progressively apply your brakes the car pulls to the left gently," he said.
In another case, a driver narrowly avoided a head-on collision with a 38-ton lorry when his car veered towards the truck when he braked suddenly. The owner later discovered that the car swerved because his suspension had collapsed.
Last night's show claimed that a new-style Mondeo suspension arm had been introduced to combat severe wear caused when a car wheel hits a bump in the road.
Watchdog first raised alleged problems with the Mondeo's steering last October and said that it has received nearly 400 complaints since its first programme. Ford say more than 400,000 cars have been sold and they have no evidence to suggest anything is wrong.
Ford also disputed the number of complaints. "We have asked the programme makers for names and addresses of all the Mondeo owners to check out each complaint. So far the BBC have only given us 150 letters," said a spokesman.
The company told Watchdog: "Ford wishes to inform viewers that Mondeo drivers and their families should be reassured that in spite of extensive investigation it has no evidence to suggest there is a design or manufacturing defect with the steering of the Mondeo."
One industry source said that it was unlikely that Mondeos were faulty. "They are mainly cars used by sales reps and tend to be designed to take a pounding. All cars tend to drift left on the left-hand side of a road which has a camber.
"There are a number of reasons why the cars tested could have behaved in the way they did. Perhaps their wheels were not aligned correctly or the car had been bounced off a kerb."