From Monday forcing someone into marriage in England and Wales will carry a maximum seven-year jail sentence under the Anti-Social Behaviour, Crime and Policing Act 2014.
The government hopes the law change, which also criminalises forcing a British national into marriage outside the UK, will protect thousands of potential victims.
May said she was proud of the UK’s role as a world leader in combating the crime, saying: “Forced marriage is a tragedy for each and every victim, and its very nature means that many cases go unreported.
“I am proud to say that the UK is already a world leader in the fight to stamp out this harmful practice with the government’s Forced Marriage Unit (FMU) working hard to tackle this terrible practice in the UK and overseas. Today’s criminalisation is a further move by this government to ensure victims are protected by the law and that they have the confidence, safety and the freedom to choose.
“This summer, the prime minister will also be hosting the UK’s first summit dedicated to galvanising action against forced marriage and related issues.”
May has joined anti-child-marriage charity Freedom, the police, the Crown Prosecution Service and a victim of the practice in a an online video for the £Freedom2Choose campaign, explaining why forced marriage has been criminalised.
The change in law was originally announced in 2012 by David Cameron, who said forced marriage was “abhorrent” and “little more than slavery“.
A forced marriage is described as one in which one or both spouses do not consent to the marriage but are coerced into it by physical, psychological, financial, sexual or emotional pressure.
The Home office said the FMU gave advice or support related to a possible forced marriage to more than 1,300 people in 2013.
As well as outlawing the practice, breaching a civil Forced Marriage Protection Order (FMPO) will be punishable by five years in prison. Freedom founder Aneeta Prem welcomed the new legislation.
She said: “In the most tragic cases, people forced into marriage become domestic slaves by day and sexual slaves by night. Today’s announcement sends out a powerful message that this indefensible abuse of human rights will be not be tolerated. Everyone should have the freedom to choose.”
Earlier this week, the NSPCC said children as young as 12 were calling ChildLine about forced marriage, with the number contacting them up two-thirds in the past year.
Dr Ash Chand, the charity’s strategy head for minority ethnic children described the law change as “a huge step forward, which we hope will deter those plotting against their own children”.
He added: “Many young people who call our ChildLine service about this issue are frightened, concerned and feel control of their lives is being wrenched from them. We have produced an animation that will help allay their fears and encourage them to contact us for help and advice.”