The Home Office is set to propose the law this summer as part of a review of prostitution laws, which could impose heavy fines or jail sentences of up to six months on men who pay for sex.
Other proposals include making it easier for prostitutes to exit the profession by providing access to clinics that treat sexual diseases and drug addiction, and offering careers and basic-skills training, counselling and financial advice.
But Cari Mitchell, a spokeswoman for the group, which has campaigned for the decriminalisation of associated prostitution laws since 1975, believes the Government is more likely to opt for tougher laws, in line with the current crackdown on anti-social behaviour.
"We are not confident that any of the proposals that benefit prostituted women will be pursued," she said. "Criminalising clients is not the way to go. It is brutal, will take the bread-and-butter money out of prostitutes' hands and increase the danger of violence for street prostitutes, who would need to take more risks by getting into cars faster because men would be more nervous of the police."
Mitchell added: "It is a complete waste of resources for police to be chasing after people who pay for consensual sex."
Mike McCall, director of operations at St Mungo's, added that criminalising men would be very difficult to implement.
In November 2002, the homelessness charity helped set up the Working Women's Crack Project, which provides accommodation for drug-addicted sex workers so they can detox and rehabilitate.
"There should be similar projects in every central London borough," said McCall. "It's important that more schemes provide emotional support as well as shelter and detox facilities.
"Any change in government strategy must ensure that drug services are responsive and flexible, because positive encouragement to get prostitutes off the streets must be given, instead of relying on police tactics. But it sounds as if the Government is seeing the bigger picture and taking a more holistic approach."
Mitchell called for the decriminalisation of associated prostitution laws to allow prostitutes to set up small businesses from home to enhance their safety. She said New Zealand had done just that and that it seemed to be working.
She added that the Magistrates' Association has long campaigned that prostitutes under 18 shouldn't be criminalised, and added: "Why shouldn't that extend to adult women who are mainly mothers trying to escape violence."