 "Britain makes forced marriages a criminal offence")
"Forced marriage is a dreadful form of abuse that will not be tolerated in our country or our communities," Cameron said in a statement.
"It leaves its victims at risk of physical, sexual and emotional abuse so it is right it is now a criminal offence for which perpetrators can face up to seven years in prison."
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The Anti-Social Behaviour, Crime and Policing Act 2014 will be enforced not only within Britain but also makes it a criminal offence to force a British national into a marriage abroad, as many youngsters are taken to weddings in their countries of origin, mainly Pakistan, India and Bangladesh.
Under the new legislation anyone found guilty of forcing a person into marriage faces a maximum jail sentence of seven years in England and Wales.
"I am proud that Britain has been at the forefront of an international effort to stamp out forced marriage," he said.
Cameron said the British government has worked closely with charities and community groups to "reach this major milestone."
He said he will hold a global summit later this year to chart measures with international partners to protect women and girls from abuse, including forced marriage and female genital mutilation.
"Marriage is a wonderful expression of love, freedom and choice, and everyone should have the freedom - now backed by the law - to choose who they marry and when," he said.
The problem largely affects Britain's immigrant communities with origins in South Asia.