Forced marriage is "little more than slavery", David Cameron said as the government announced plans to make it a criminal offence.

Parents who force their children into a marriage will face jail in what Mr Cameron said was a "clear and strong message" that the practice would not be tolerated.

Campaigners have warned that criminalising forced marriage altogether could deter victims from coming forward but Mr Cameron said an extra £500,000 of funding would help identify and support those affected.

The Prime Minister said: "Forced marriage is abhorrent and is little more than slavery. To force anyone into marriage against their will is simply wrong and that is why we have taken decisive action to make it illegal.

"I have listened to concerns that criminalisation could force this most distressing issue underground.

"That is why we have a new comprehensive package to identify possible victims, support those who have suffered first hand and, indeed, prevent criminality wherever possible.

"We have spent time with those who work tirelessly to raise and address this issue and I want to send a clear and strong message: forced marriage is wrong, is illegal and will not be tolerated."

Under the plan, forced marriages will become a criminal offence in England, Wales and Northern Ireland. The scale of sentences available will be set out in the legislation.

Home secretary Theresa May said: "It is the right of every individual to make their own choices about their relationships and their future.

"Forced marriage is an appalling practice and by criminalising it we are sending a strong message that it will not be tolerated.

"But we know that legislation alone is not enough and we will continue to work across government and with frontline agencies and organisations to support and protect victims."

Andrew Flanagan, chief executive of children's charity NSPCC, said the change would need to be monitored to ensure the problem was not pushed further underground.

He said: "Forced marriage is ruining young lives right here in the UK.

"Very young girls and boys are being coerced or even forced through violence and intimidation to marry spouses sometimes twice their age. This is child abuse plain and simple.

"It's welcome that the government is focusing attention on this major issue in child protection and it's right that we are not afraid to tackle this problem head on.

"The NSPCC has always had concerns that a specific criminal law may risk making this abuse harder to uncover because victims could be afraid of criminalising their family if they speak out.

"So it's positive that the government has listened and is looking at wider measures to support young people and ensure they will not be deterred from seeking help.

"The government must now work closely with professionals such as social workers, teachers and police to make sure any law works in the best interests of the victims.

"And if this law is used we want to make sure the response to the crime is right, the right people are prosecuted and, crucially, young people feel supported and able to come forward when they need help.

"Close monitoring will be required to ensure that these changes are making a real difference and don't force this problem further underground."

In a sign of how widespread the problem is, the government released statistics that showed the cross-departmental Forced Marriages Unit had been involved in almost 594 cases from January to May this year.

Some 45% involved victims under the age of 18, 87% of all cases involved a female victim and 46% had Pakistan as the country of origin.

London had 20.9% of cases, the West Midlands 16.7%, South East 10.4%, Yorkshire & Humberside 5.3%, North West 5.1%, South West 2.6%, East Midlands 2.2%, Scotland 1.9%, Wales 1.4%, North East 1.4%, East of England 1.2% and Northern Ireland 0.2%.

Freedom, a charity which campaigns against forced marriage and dishonour violence, welcomed the crackdown.

Founder Aneeta Prem said: "Forced marriage is an indefensible abuse of human rights and can result in abduction, serial rape and murder.

"Criminalising forced marriage will send out a powerful message to people that this practice is unacceptable in England and will be dealt with severely."

Lord Toby Harris, a Labour peer and chair of trustees for Freedom, said: "I hope the government put the necessary Bill before Parliament as soon as possible, along with measures to support organisations like Freedom who work to advise and support victims and potential victims."