Charities have accused the government of failing in their first test to help victims of trafficking.
They have criticised the decision not to adopt an EU directive which they say will give victims better protection and lead to more trafficker prosecutions.
The Home Office says the UK already has ample measures to help victims.
The European directive on preventing and combating trafficking is expected to be approved in the autumn. Denmark and the UK are not signing up to it.
But charities, including Anti-Slavery International, said the trafficking of people throughout Europe demanded a cross-border solution and that the UK should step into line with other EU states.
Supporters of the proposed directive say it is better than the existing European convention which sets out how best to deal with the criminals and to protect the victims.
It aims to establish common standards across member states for the prosecution of traffickers - but it also proposes greater protections for victims who have been involved in crime in the country they are held in.
This could mean that some people who are trafficked into criminal enterprises in the UK, such as the sex trade or cannabis farming, could not be charged over false immigration papers forced on them by the gang responsible for their move.
Klara Skrivankova, of Anti-Slavery International, said: "Despite significant positive steps, the Government cannot become complacent and say that the UK is already doing enough.
"Without international cooperation the government will lose the battle with the traffickers. By choosing not to opt in to the directive the Government is failing in its efforts to combat this transnational crime."
A Home Office spokesman said the directive offered no new benefits to the UK, but ministers would review the position once the directive had been agreed.
"Human trafficking is a brutal form of organised crime and combating it is a key priory of the new government," said the spokesman.
"Opting in now would also require us to make mandatory the provisions which are currently discretionary in UK law. These steps would reduce the scope for professional discretion and flexibility and might divert already limited resources."
But Christine Beddoe, of Ecpat UK, a charity that helps children who have been abused and trafficked, said: "The government claims that it wants to make the UK a hostile environment for traffickers but this cannot be achieved unless the whole EU takes a common approach."
In June a coalition of charities published a report that said the system for handling victims in the UK was "not fit for purpose".
The Crown Prosecution Service has separately launched a public consultation on trafficking in which victims will be asked their views.