Police in major cities in the United States are criminalising women who carry a stock of condoms, making sex workers and their clients less likely to use them and increasing their risk of contracting HIV, says Human Rights Watch.

A new report compiles evidence from sex workers in four major cities – Washington DC, New York, Los Angeles and San Francisco.

Interviews with more than 300 people show that those most at risk of HIV, such as sex workers and transgender women, are afraid to carry condoms in case they should be stopped and searched by police.

"Some women told Human Rights Watch that they continued to carry condoms despite the harsh consequences. For others, fear of arrest overwhelmed their need to protect themselves from HIV, other sexually transmitted diseases and pregnancy," says the report.

One of the interviewees, Carol F, a sex worker in Los Angeles, told HRW she had been arrested partly on the basis of carrying condoms. "After the arrest, I was always scared … There were times when I didn't have a condom when I needed one, and I used a plastic bag," she said.

Prostitution is a crime in 49 states of the US. Law enforcement agencies view the possession of multiple condoms as evidence that could help a court prosecution for a breach of the law.

But HRW says they are impeding efforts to prevent the spread of HIV, costing millions of dollars. "Enforcement … must be compatible with international human rights law, and governments should ensure that police policies and practices do not conflict with equally important public health policy imperatives, including those designed to curb the HIV epidemic," says the report.

The report is published before the International Aids Conference, which begins in Washington DC at the weekend.

Washington, which has an HIV prevalence of over 3%, has taken strong measures to tackle an Aids epidemic as serious as that in some parts of Africa.

The District of Columbia distributed 4 million condoms in 2010 through a programme it calls "The Rubber Revolution".

But sex workers and transgender women told HRW that they are searched and questioned by police if they are carrying multiple condoms.

Their accounts were supported by Jenna Mellor, director of outreach for HIPS (Helping Individual Prostitutes Survive).

Mellor runs a mobile outreach van that provides condoms, clean syringes, and other harm-reduction materials to sex workers several times per week.

"Fear of taking condoms is a real problem," Mellor told HRW. "Clients take fewer condoms than they need because they fear the police. They also hide condoms in their clothes, their wigs, their cleavage, in order to avoid being hassled by the police."