The Met faces a "major challenge" to drive up rape convictions in the capital, according to its most senior cop.

Met commissioner Sir Paul Stephenson said police were hindered by the fact it was "unbelievably difficult" in some situations to gather the evidence needed for a successful conviction.

He spoke as statistics were released showing Lambeth had the highest number of reported rapes in London in the last financial year. Lambeth was also named a hotspot for gang rape.

In 2009-10 there were 186 incidents reported compared with 145 in 2008-9. At the same time rape sanction detection in the borough fell by 21 per cent to 16.47 per cent, against a target of 35 per cent.

Sir Paul said this did not prove for certain incidents of rape were on the rise. He said he hoped it was because the public had more faith in police, and the systems in place to help them report crimes, including the Havens, and specialised sexual assault referral units across London.

He said: "They can be unbelievably difficult offences to investigate, but we have had many positive outcomes, especially in situations of stranger rape."

He said the police needed to always look at the situation from the victim's perspective and make sure dealing with the police was a positive experience. He said it was very important victims continued to come forward.

He said the decision to centralise the capitals' rape investigation team after high profile failures to investigate serial rapists Kirk Reid, who struck in Streatham Park, and taxi driver John Worboys, whose victims included a Herne Hill resident, was correct and would drive up results and standards.