The letter has been sent to 40,000 new Mondeo drivers in the UK and 4,000 in Ireland, as well as dealers, outlining correct procedure for applying the handbrake.
Ford will also be revising the handbrake section of the car's driver's manual for future deliveries. The firm denies there is a fault with the handbrake and claims incidents have only occurred where the handbrake has been applied incorrectly.
Glamorgan based stationery company Staedtler has recently added three diesel Mondeos to its fleet of 30 cars. One was involved in an accident after it was parked.
Finance director Glyn Davies said: 'One of our drivers parked his car at a petrol station and went to get some change for the car wash. When he came back he saw the car rolling down a slope into a recovery truck. There were four witnesses who saw the incident and confirmed that the handbrake was raised.'
Ford has also received other reports of incidents but could not say how many.
The Ford letter to customers says: 'Some owners of new Mondeos are uncertain about how hard they should apply the handbrake when leaving their car parked.
'In a small number of cases, owners have reported that their car moved following parking and investigation of these cases revealed that the handbrake had not been fully applied.'
A spokeswoman said: 'The whole handbrake design and manufacture has been scrutinised and no fault has been identified. We have responded to the problem as quickly as we could and sent out advice on how to solve it.'
In its letter, the manufacturer recommends depressing the brake pedal firmly before pulling the handbrake up 'smartly to its fullest extent'.