The new legislation is aimed at protecting women from pimps
Home Secretary Jacqui Smith has been accused of back-tracking on a pledge to criminalise men who pay for sex with women forced into prostitution.
The government has changed the wording of legislation designed to protect victims of pimps and traffickers.
Women's charity Eaves said the law - due to receive its third and final reading - had been diluted.
But the Home Office said there was "no U-turn" and the law would protect women subjected to threats or violence.
Clause 13 of the Policing and Crime Bill was originally drafted to create an offence of the purchase, or attempted purchase, of sexual services from anyone "controlled for gain by a third party".
However, the home secretary now proposes to replace "controlled for gain" with "subjected to force, deception or threats".
The Home Office spokesman said: "There is no U-turn. We have made a slight change to the wording of the legislation to stop the offence applying more widely than we intended it to."
Critics had warned the law would be difficult to enforce and could unnecessarily take income away from women who sell sex voluntarily.
But Eaves said the new wording was too narrow, and is calling for criminalisation of all forms of demand.
The amendment... doesn't cover the exploitation of vulnerability which commonly occurs in cases of grooming of women and girls into prostitution
It said that by making the change, Ms Smith's amendment would be likely to deny justice to British women in particular, to whom trafficking legislation does not usually apply.
Eaves runs the Ministry of Justice-funded Poppy Project, which provides accommodation and support to women trafficked into the UK for the purposes of sexual exploitation.
It warned the new wording might remove protection from those who are psychologically pressurised into selling sexual services.
Spokeswoman Helen Atkins told BBC Radio 4's Today programme: "The revised amendment narrows the offence so it is arguably narrower than the international trafficking definition which requires 'force, fraud or coercion'.
"The amendment the Home Secretary has put forward doesn't cover the exploitation of vulnerability which commonly occurs in cases of grooming of women and girls into prostitution and the psychological control that often comes with it," she said.
The charity said it wanted similar wording as given in the Forced Marriage Act of 2007, which requires coercion by threats or other psychological means.
"This would capture the more insidious methods used to control women," she said.
Two other groups - Rights of Women and the Christian charity CARE - also said psychological manipulation should be taken into account.
The Home Office insisted the law would protect women who were subjected to "physical or psychological threats, deception or force".
"Our position is, and has always been, clear: buyers who pay for sex with trafficked women will be breaking the law," the spokesman said.
Nicky Adam, of the English Collective of Prostitutes, said the proposed changes were an improvement.
She said: "The change is an acknowledgement that there is a difference between prostitution and violence."
But she also said the ECP believed the whole clause should be abolished, because there were existing laws to deal with situations where a woman was subjected to violence or coercion.
"The 'controlled for gain' clause was so wide that it could have been used against anyone working with another person - even someone working with another prostitute for safety," she added.
"It would have been used to criminalise women working independently and collectively, forcing women to work on their own, thus making them much more vulnerable to attack."