Nearly 100 officers from three forces covering the areas are conducting a joint investigation, codenamed Operation Monarch.
Commander Paddy Tomkins, of Scotland Yard, said the man could strike again. The attacks include the sexual assault of two girls of 14 and 15 in a wood and the rape of a 15-year-old.
The joint inquiry began in November in Essex after three attacks in Southend and Westcliff were linked. Subsequent investigations showed a rape in north-west London and an earlier attack in Brighton were by the same man.
Mr Tomkins said: "This man is a prolific attacker. We are obviously concerned that he may strike again. It also entirely possible that this man is responsible for other attacks that may not have been reported to police."
Detective Chief Superintendent Lee Weavers, of Essex Police, added: "Detailed analysis and investigation has revealed strong links between these 10 offences in terms of the suspect's speech, his behaviour patterns and the duration and method of attack."
He is a described as white, possibly tanned, aged 20 to 30, of medium build and 5ft 8in to 5ft 10in tall. He has short brown hair, sometimes described as wavy on top and shorter at the sides. He speaks with a southern accent, and sometimes wears a cream or white knitted shirt with black stripes, similar in style to a rugby shirt.
The attacks were:
r A serious sexual assault on a 22-year-old at 3pm at her home in north- west London on 29 October this year;
r Sexual assault on a woman aged 21 in a street in east London at 10.40pm on 28 October;
r Rape of a 26-year-old woman in north-west London at 4am on 13 September ;
r An assault on a 16-year-old in Southend, Essex, on 12 September at 9.45pm. The attack was interrupted by the victim's boyfriend;
r A serious serious sexual assault on a 22-year-old woman, also on 12 September, in Westcliff, Essex, at about 11pm;
r The rape of a 15-year-old girl in Eaton Road, Southend, at 3am on 31 August;
r The rape of an 18-year-old in Brighton at about midnight on 29 August;
r A serious sexual assault on a 17- year-old girl on waste ground in north-west London at about midnight on 23 May.
r Sexual assault on a woman in an empty garage in north London at 10.15pm on 19 May;
r A sexual assault on two schoolgirls, aged 14 and 15, in woodland at Rayleigh, Essex, at 5pm on 12 August 1997.
Anyone with information should call 0800 389 2873.
In an unrelated development, police are to ask a court to impose a curfew on a freed rapist who returned to live near the scene of his crimes.
In what is believed to be the first case of its kind, police will seek an order under new laws against Michael Gordon, 35, from Manchester, dubbed the Bedsit Beast after being jailed for 12 years in 1990 for raping two students at knifepoint.
A stipendiary magistrate in Manchester will next week hear submissions by Greater Manchester Police that Gordon, freed under licence earlier this year, should be subjected to a 10pm to 7am curfew.