Labour's ban on conversion therapy could cost businesses and taxpayers as much as £100million over a decade, according to the Government's own figures.
An official report admits the new legislation – which outlaws abusive attempts to change people's sexuality or gender identity – will have a 'negative' impact on private firms and public sector bodies who will have to train staff on its workings.
The price tag will rise still further as the costs to the police, courts and prisons of implementing the Conversion Practices Bill have not yet been calculated.
But the Impact Assessment also claims the law will be worth a massive £783m to society over 10 years, on the grounds that it will mean fewer transgender and gay people suffer from depression.
Last night critics – who fear the ban will risk parents being jailed for trying to persuade their children not to transition – cast doubt on the figures.
Maya Forstater, Chief Executive of women's rights charity Sex Matters, said: 'The idea that the conversion practices bill could contribute nearly £800m to the economy is pure fantasy based on shaky evidence, while the cost of the bill has been grossly underestimated.
'The largest costs such as the cost of police and court time, social workers and children being taken into care and the impact on families have not been counted.
'Both the projected benefit and cost are unserious figures that are not fit to be anywhere near such a substantively dangerous bill.'
Activists calling for a ban on conversion therapy at Brighton Pride in 2023
Shadow Equalities Minister Joy Morrissey added: 'At a time when businesses across the country are already feeling the pinch from Labour's disastrous economic policies, it would be wrong to impose more than £100 million in additional costs through this draft legislation.'
The Impact Assessment admits the evidence base for conversion therapy is 'relatively limited' and that incidents are 'not currently captured in any official statistics', so estimates of its prevalence 'heavily rely on self-selected samples'.
It says the Cabinet Office used a survey commissioned by lobbying group Stonewall, which asked people if they had been told to take 'purifying' substances or given 'pseudo-scientific' counselling sessions, to calculate that between 75,000 and 93,000 people a year in England and Wales are victims of conversion practices.
The study estimates that up to one in three have depression as a result of their experiences, going on to say that improving their quality of life will lead to a 'monetised benefit'.
It states: 'We expect that the number of people experiencing depression as a result of conversion practices on the basis of sexual orientation and/or transgender identity will be reduced with this option. Our estimate of the 10-year net present value of this benefit is £783 million.'
The Impact Assessment also says that 'there may be positive economic benefits associated with corresponding reductions in sick leave from work'.
But the report goes on to admit: 'The overall impact on businesses from these measures is likely to be negative.
'We expect that there will be one-off familiarisation costs to organisations in the private education and private healthcare sectors associated with the legislation. There are also small familiarisation costs to religious groups and the LGBT+ voluntary sector.'
The introduction of the law is predicted to cost taxpayers £45.8m for the police, Crown Prosecution Service, courts, NHS, education sector and social services to train staff, while a Victim Support Service will cost another £240,000 over three years.
In addition, although it is expected that 'costs will be incurred within the Prison and Probation Service and Criminal Legal Aid divisions', the report says 'impacts on the wider justice system' are still being assessed with the Ministry of Justice'.
One-off 'familiarisation' costs are forecast to reach £13.6m for the private sector.
A table showing the total costs to business and the public sector over 10 years includes a high estimate of £103.3m alongside a best estimate of £59.7m.
