Ministers are accused of "betraying" human trafficking victims by delaying ratification of a European treaty.
Thousands of women in the UK are thought to be trafficking victims
Shadow home secretary David Davis said the government had signed the treaty to "great fanfare" last year - but as yet had no plans to ratify it.
The treaty allows victims time to recover and consider testifying rather than being immediately deported.
The Home Office says it has introduced anti-trafficking laws and will ratify the treaty as soon as possible.
The government says research suggests 4,000 women in the UK are working in areas of sexual exploitation and may have been trafficked - or moved by force or deception with the aim of exploiting them.
The Conservatives say the conviction rate for trafficking is down 40% on the previous year and there are still no convictions for labour exploitation.
Mr Davis outlined various measures the Conservatives would take, such as increasing the number of places at safe houses and changing rules to allow support groups to help trafficked children aged 16 to 18.
And he said an anti-trafficking campaign involving every police force in the UK and Ireland - Operation Pentameter - should be made permanent.
The Conservatives say the European Convention Against Human Trafficking, signed by the government in March 2007, could help increase convictions for human trafficking.
But Mr Davis said while the convention had been signed "amidst great fanfare last year", the government "doesn't even have a plan or timetable for ratification".
He added: "In the meantime, this dreadful problem continues, the traffickers increasingly operate with impunity and we are betraying the victims of this appalling trade, including some of the most vulnerable in our society."
He added: "We call on the government to honour its commitment to become a party to the convention without delay."
The convention covers a range of measures, including 30-day residence permits to victims to allow them to recover from their ordeal and reflect on whether they will help police prosecute offenders.
There had been some fears this could be open to abuse by people making false claims of being trafficking victims to remain in the country.
A Home Office spokeswoman said the government had already introduced "wide ranging" anti-trafficking laws as well as setting up the Serious Organised Crime Agency and the UK Human Trafficking Centre.
She added: "We have signed the Council of Europe Convention and will ratify as soon as we can. We are determined to get the arrangements right before we ratify.
"This involves widespread consultation and primary and secondary legislation. This is not unique to the UK. Most other EU countries are in a similar position."
UK Independence Party MEP Godfrey Bloom said the treaty would do little to help because the EU had made moving across borders "so easy".
"I would have thought that if people really do want to do something about human trafficking, then they'd put proper border controls in instead," he said.
The treaty is due to come into force in 10 countries in February.