Peers are due to vote today on a parliamentary bill amendment which would create two new laws criminalising modern-day slavery and forced labour.
Human rights campaigners yesterday called on the Government to introduce the legislation to protect victims.
The House of Lords is expected to vote on the new offences following a debate on the Coroners and Justice Bill.
More than a thousand people, including maids, fruit pickers and factory workers, are forced to work as slaves in the UK, according to Anti-slavery International and Liberty.
Many of them are exploited by relatives or employers who take away their passport, treat them like prisoners, and make them work without pay.
Trading in slaves was made a criminal offence in the 19th century and offenders can be prosecuted for trafficking people for exploitation, false imprisonment, kidnap and breaking employment laws.
But there are no laws which specifically target holding a person in servitude or subjecting a person to forced labour.
Hundreds of people are thought to be subjected to slavery in the UK without being trafficked - and cases of forced labour are difficult to prosecute when trafficking is difficult to prove.
Aidan McQuade, director of Anti-Slavery International, said: "To not have a law to protect people from forced labour is comparable to Britain not having criminalised torture.
"Getting the police to prosecute those who hold people in modern day slavery is extremely difficult because of the lack of a clear offence criminalising this practice.
"Forced labour will remain a reality in the UK unless adequate legislation is put in place and enforced.
"The existing legal provisions fail to protect victims or ensure that the perpetrators of these crimes are brought to justice."
Shami Chakrabarti, director of Liberty, said: "In an age when new criminal offences have flown out of Westminster like confetti, the lack of an effective anti-slavery law is a gaping hole in the protection of the vulnerable.
"We urge parliamentarians of all stripes to join together in supporting this amendment and honouring the tradition of William Wilberforce."
The new laws would make holding someone in servitude an offence punishable by a maximum of 14 years and subjecting someone to forced labour punishable by up to seven years.
Legal advice given to the charities by former director of public prosecutions Sir Ken Macdonald QC and Helen Mountfield, both of Matrix Chambers, said the new laws were also necessary to bring the UK in line with the European Convention on Human Rights.
Article Four imposes an obligation on the UK to protect people from slavery, servitude and forced labour.
Sir Ken wrote: "We do not consider that the existing provisions of the English criminal law provide effective protection and penalties for servitude and forced labour.
"The introduction of such offences is necessary, both to protect the victims of serious abusive crime, and in order to avoid findings by the European Court of Human Rights against the United Kingdom of violations of Article Four."
There are an estimated 12.3 million people in forced labour worldwide with 360,000 in industrialised countries such as the UK, according to Anti-slavery International.
The new laws will state that servitude occurs when somebody is subjected to forced labour, severely restricted in their freedom of movement and the choice of where they live.
Forced labour occurs when somebody is made to work through threats, deception, fraud, coercion or debt bondage.Reuse content