The force would not comment on the nature of the allegations.

Mayor's reaction

Boris Johnson, London's mayor, said the allegations were "extremely serious and need to be thoroughly investigated".

Two newspapers, The Times and the Daily Mail, and Britain's Sky News television channel reported that officers had allegedly held suspects' heads under water.

The Metropolitan Police said a police employee had raised concerns about "a small number of officers" in Enfield, and the case had been referred to the watchdog in April.

The force said that, if allegations of wrongdoing were proved true, "the strongest possible action will be taken".

G20 allegations

Britain is signatory to the United Nations Convention Against Torture, which requires that governments prevent torture anywhere under their jurisdiction.

London police officers are also facing allegations that they used excessive force during protests against the G20 summit in London in April.

The IPCC is investigating several incidents, including the death of Ian Tomlinson, a newspaper vendor, who collapsed and later died after he was pushed to the ground by police.

Results of an initial autopsy suggested Tomlinson died of a heart attack, but a second examination said he had suffered internal bleeding.