Wiltshire detectives working on the murder of Swindon woman Sian O'Callaghan are speaking to officers in Avon and Somerset who are still investigating the death of Wiltshire woman Melanie Hall.

Miss Hall, 25, from Bradford on Avon disappeared from a nightclub in nearby Bath in 1996. Detectives have been alerted because of similarities between the two cases.

The body of hospital worker Melanie Hall, 25, was found near the M5 in Gloucestershire thirteen years later.

The hunt for the missing night–clubber Sian O'Callaghan turned into a double murder investigation last night as detectives disclosed that they had found her body and suspected another was nearby.

The discovery came six days after Miss O'Callaghan, 22, was last seen alive leaving a club. Police said they believed they had found her body close to the Uffington White Horse beauty spot in Oxfordshire. In an extraordinary twist, officers then said they had been told about a second as yet unidentified body, which they have yet to find.

The man arrested is believed to be 47– year–old Chris Halliwell, a taxi driver. The suspect is being held on suspicion of kidnap and double murder.

Det Supt Steve Fulcher, who is leading the inquiry, said: "A 47–year–old man from Swindon is in custody, having been arrested for kidnap and two murders.

"The location of two bodies have been identified to me by this individual, one of whom has yet to be identified formally, but I am quite clear is Sian." Mr Fulcher added that Miss O'Callaghan's parents Mick, 51, and Elaine, 48, had been told about the discovery and were deeply distressed.

However, the detective would not be drawn on the identity – or even the gender – of the other body. Reports last night claimed it was that of an unnamed woman killed in 2001.

The developments came after the arrest of Mr Halliwell at a taxi rank outside an Asda store in north Swindon yesterday morning. Officers towed away his green Toyota Avensis. According to reports he had gone voluntarily to a police station on Wednesday to give a sample of his DNA after officers said they were looking for a Toyota Avensis cab.

Mr Halliwell, a divorced father–ofthree, lives in Swindon with his partner, Heather Widdowson. Their end–of–terrace house was cordoned off yesterday. Forensics officers were seen searching the property and working in the back garden.

Miss O'Callaghan, an office administrator, was last seen alive leaving Suju nightclub in the early hours on Saturday. CCTV footage showed her shortly before 3am and her mobile telephone was tracked to Savernake Forest, near Marlborough, about 30 minutes later.

Her disappearance resulted in a major police search and more than 400 volunteers joined the hunt.

Her body was found more than 20 miles away near the Uffington White Horse, a 374ft long ancient chalk figure carved into a hillside in Oxfordshire.

Up until midday yesterday, police hunting Miss O'Callaghan believed that there was still a chance that they would find her alive.