Michael Mannion, 35, from London, has told Indian police that he saw barman Samson D'Souza on top of 15-year-old Scarlett Keeling hours before she was found dead on a beach in Goa.

But Mr Mannion claims that Indian authorities are now preventing him from leaving the country.

The body of Scarlett, who had been on holiday from her home near Bideford, North Devon, was found on Anjuna beach on February 19. She had been raped and murdered. Police in Goa have said she was given a cocktail of Ecstasy, cocaine and LSD.

D'Souza, 28, was the first man arrested by Goan police in connection with the death. He is in police custody suspected of rape. A second man, Placido Carvalho, aged between 30 and 35 years old, is also in police custody having appeared before a court on suspicion of drugging Scarlett and assisting in the murder by that act.

The case has now been handed over to the Indian Central Bureau of Investigation (CBI).

Mr Mannion's lawyer, Vikram Varma, said Goan police had placed his client on a "look out circular" which effectively prevents him from leaving the country. Mr Mannion, a carpenter, said he needs to leave Goa so he can return to the UK to look after his sick father.

Speaking from Goa, he said: "I've been placed on a look out circular. It's basically an order issued to all immigration posts, such as airports and ferry ports. If I try to leave the country I'm stopped and sent back.

"They (the Indian police) keep telling me I'm the key witness, I've given a deposition to the police but now the case is being handed over to the CBI and I've got to give them a deposition also, and we don't know how long that will take.

"I understand the need to follow the investigation thoroughly and I've been willing to help, but if this is the way witnesses are treated it could dissuade other people from coming forward."