A millionaire businessman and his family, including a three-month-old baby, have disappeared, provoking an international police inquiry.

Scotland Yard detectives have become increasingly suspicious about the disappearance of the Asian family from west London who have not been seen for 45 days. They are investigating several letters, including one written from France, that purport to be from Amarjit Chohan, 46, the head of the family, claiming he had decided to go back to India with his sons, Ravinder, aged three months, Devinder, aged 18 months, his wife Nancy, 25, and her mother, Charanjit Kaur, 51.

But, despite the letters, Mr and Mrs Chohan's passports are still in Britain, their bank accounts remain untouched, no relatives or friends have been contacted, and all the children's possessions are still at the family home in Heston, Hounslow.

Five days after the Chohans were last seen, the family car – a blue Ford Escort, registration number S840LJH – was involved in a minor accident with another vehicle near Southampton. But the two people in the car – who gave false names and addresses – were a white and a black man.

The initial police inquiry has been taken over by Scotland Yard's Serious Crime Directorate. Interpol, the international police agency, and the Indian authorities, have been alerted.

Mr Chohan is a well-known figure within the Asian community in Southall and runs a fruit and vegetable import company, called CIBA Freight. The firm, which imports products from Kenya and Uganda, has an annual turnover of £45m. Mr Chohan also owns five properties worth about £2.5m and has a substantial amount of money in his bank accounts.

Despite his wealth, the family has a modest lifestyle and Mr Chohan drives a battered car, although he has made generous donations to various events held in Southall. The family's disappearance has shocked staff at CIBA Freight, family and friends.

The first indication of trouble came on 14 February when a letter signed in Mr Chohan's name was sent to his staff saying that he "had enough" and was returning to India, where he was born. Another letter was written two days later to the company's operations manager with a similar message. Like all the letters since the family's disappearance it was written on a computer, which provoked suspicion because Mr Chohan's correspondence was always hand written. The signature is being examined by handwriting experts to discover if it is forged.

His wife, mother-in-law, and two boys were last seen at their home on 16 February. Mr Chohan was seen on the same day driving his Escort in Wiltshire and Hampshire.

On 21 February the same vehicle was involved in an accident, but has not been seen since. A month later, on 24 March, a letter with a postmark from Calais was sent saying that the family were in France and intended to go to India.