Brooke Magnanti, the author better know by her alias, the fictional call girl Belle de Jour, has warned that the policing of sex workers in the runup to the Olympics is putting their lives at risk.

Magnanti, whose memoirs were turned into a television series starring Billie Piper, has given her backing to the campaign group Stop the Arrests, which has written to the mayor of London, Boris Johnson, expressing fears that police tactics are compromising the safety of sex workers.

Other signatories to the letter include the Labour MP John McDonnell and Jenny Jones, chair of the Green party. It claims the raids are part of "clean-up efforts" targeting sex workers and sex establishments, particularly in east London, in the runup to the Olympics and calls on the mayor to intervene.

Keeping a brothel is illegal under the Sexual Offences Act 1956, but how the act is policed varies from force to force. Sex workers claim the act is being invoked far more stringently in the runup to the 2012 Games – even though there is no evidence there has been an increase in prostitution.

"These efforts are partly a response to the claim – made by governments, charity organisations and campaign groups – that the Olympics will lead to an increase in trafficking for prostitution," the letter says. "This is despite the fact that there is no evidence that large sporting events cause an increase in trafficking."

Sex workers claim the police tactics are stopping them from reporting crime. Last December in the east London borough of Barking and Dagenham, a violent gang carried out a series of robberies on brothels at knife-point. Sex workers at the brothels claim they were deterred from reporting the attacks after police threatened them with prosecution.

Those backing the campaign say raids and closures of premises have resulted in arbitrary arrests, detention and deportation of people working in the sex industry. The letter says: "This creates a climate of fear among workers, leaving them less likely to report crimes against them and more vulnerable to abuse and exploitation. It is an inadequate response to sex work and to trafficking."

Magnanti said she was backing the campaign because history suggested heavy-handed tactics against the world's oldest profession always failed. "I support the campaign because it's time for us to realise, no matter how we feel about sex work, that making criminals of the people in it is no solution," she said. "Historically, prohibition has never been an effective strategy and often causes more harm than it stops. London is a truly international city and must lead the way with compassion and human rights for all in our society."

Magnanti has also tweeted on the issue, calling on others to add their name to an online petition and quoting former Olympics minister Tessa Jowell, who has admitted "intelligence suggests we are unlikely to see large-scale trafficking".

The Metropolitan police disputes claims that it has increased the number of raids on brothels in Olympic boroughs. But Silence on Violence, a report by Andrew Boff, a Conservative member of the London assembly, found evidence that there had been an increase in raids on brothels in Olympic boroughs.

The report noted that in the 18 months running up to March, police had closed around 80 brothels in Newham.

"All data I have been provided with shows an increase in the Met's focus on targeting sex workers in the runup to the Olympics," Boff notes. "However, there is no evidence that there has been a corresponding increase in the number of sex-trafficked victims in London, nor in the number of complaints from residents regarding brothels."

Jones questioned the Met's tactics. "The Met seems to be slipping back in its understanding of civil liberties generally, but especially in its treatment of some of the most vulnerable people in our society," she said. "London's sex workers need protection, not persecution."

Georgina Perry, manager of Open Doors – a sex-worker health project in Hackney – and another signatory of the letter, said outreach work, conducted at a sauna and flat used by sex workers, had convinced her there was increased pressure from police. Perry said the Met risked repeating the mistakes of other forces around the world. "Every major sporting event sees the same reaction from the police, which puts women in danger. But there is no evidence base to support their actions."

The letter was drafted by the x:talk project, a grassroots organisation that provides education to sex workers. Ava Caradonna, spokeswoman for the project, said it welcomed the broad support its campaign had received. "We hope the campaign will create a space among policymakers, the police and the general public for a sensible, evidence-based discussion on the question of policing and sex work."