In a major setback for the Government's plans, Commander Allan Gibson, of the Metropolitan Police's Human Trafficking Unit, questioned how effective such a crackdown could be.
Home Secretary Jacqui Smith announced last month that men would be committing a crime if they paid for sex with a woman who has been trafficked into the UK or is working for a pimp, even if they did not know she had been forced into prostitution.
The measure - part of a bid to reduce demand for trafficked women by targeting the men who pay for sex - was criticised by prostitutes, who argued it will force the trade even deeper underground.
In evidence to the Commons Home Affairs Select Committee, Mr Gibson said: "Speaking personally, I think that is going to be very difficult to enforce."
Mr Gibson told the committee that it was very difficult even for police to estimate the numbers of women trafficked into the UK for prostitution or precisely which ones were working against their will.
Over the past two years, his unit has dealt with 54 cases, and had a further 157 cases referred to it by other branches of the Met, he said.
Committee chairman Keith Vaz told minister for women Harriet Harman: "(Commander Gibson) says it is very difficult to enforce a situation where a man is expected to ask a prostitute whether or not she has been trafficked and even if he gets a negative answer he is still to be prosecuted.
"The police themselves... feel that the new proposals are unenforceable."
But Ms Harman said: "We have to address the demand side because this trade wouldn't be happening if men weren't buying sex...
"The men who are handing over the money that makes these women vulnerable have got to be made answerable for what they are doing to create this trade."
Shadow home secretary Dominic Grieve said later: "Yet again, the Home Secretary's rhetoric is defied by reality.
"The Government wants to rush through new criminal laws without any consideration as to whether they will work.
"In the meantime, it neglects the basics of law enforcement - funding for the Met's human trafficking unit has been slashed, whilst the conviction rate for trafficking for sexual exploitation has plummeted."