A total of 19 women from Malaysia were rescued by police in Britain on Wednesday as part of a major crackdown on sex trafficking, officers said.

Twelve people were arrested in a series of dawn raids in and around London and Britain's second city of Birmingham, west central England.

The suspected ringleader and his partner - believed to be of Vietnamese origin - were among those detained on suspicion of "trafficking for the purposes of sexual exploitation and conspiracy to control prostitution for gain", said London's Metropolitan Police.

The force's Clubs and Vice Unit coordinated the raids, which found 13 women at a suspected brothel in west London and another six in Birmingham.

A Metropolitan Police spokesman told AFP the women were from Malaysia and that "in all likelihood" those arrested all came from Vietnam.

Chief superintendent Ian Dyson, head of the Clubs and Vice Unit, said earlier: "This has been a very successful operation with the largest number of potential victims rescued.

"I hope this morning's operation sends out a clear message to the organisers of this vile trade that we will find them and we will seize their assets."

15-year-old girl

The women have been taken to a special reception centre where they will be given access to health professionals, interpreters and specially trained sexual offences investigators.

Those arrested - 11 men and one woman - are in custody awaiting questioning.

More than 50 officers were involved in the raids as part of Operation Pentameter, a national campaign launched earlier this year to tackle sex trafficking.

Some 44 women, aged mainly between 18 and 22, and two girls aged 15 and 17, have been found since the operation started. They originated largely from eastern Europe and the Balkans as well as Thailand, China and Brazil.

In a previous high-profile operation, 19 foreign women, largely from eastern Europe and Asia, were found at a brothel in Birmingham in September last year. Three people were later charged and are awaiting trial.