Leading trade union and employment rights law firm Thompsons says the women, who provide vital public services in the capital, could lose up to £20,000 in compensation.

Workers such as cleaners, cooks and carers in sectors like local government are being urged to think twice before they fall into the trap of going with the lawyer who shouts loudest when it is the trade unions who will deliver them the best all round deal.

No-win no-fee solicitors, who have begun to operate in London, charge up to 30 per cent of any equal pay compensation they win for women. Workers who use lawyers available free through their union will keep all their money.

And once employees have signed up with non union lawyers they run the risk of being hit by financial penalties if they decide they don’t want to proceed with the legal action or switch to their union’s lawyers.

Some no-win no-fee lawyers “fine” such women up to £500 for each six-month period regardless of the amount of work they have done on the case.

And there is a crucial difference between what non union lawyers can get and what the unions deliver. Both take claims for back pay, but only the unions make sure that the employers deliver equal pay for the future.

Where employers refuse to talk, or make unacceptable offers, unions aren’t afraid to take legal action to force them to play fair.

Unions are already in court in hundreds of equal pay cases in London on behalf of women in local government. And there will be more if councils refuse to negotiate.

Thompsons, the most experienced union law firm in the UK, believes that some no-win no-fee lawyers are failing to tell low-paid women that they can get legal support free through their unions.

“The professional rules governing solicitors are clear. The availability of free help should be explained to the women,” said Thompsons’ Head of Employment Rights Victoria Phillips. “Some firms give the proper advice, but others often fail to do so,” she said.

“No-win-no-fee lawyers are not knights in shining armour rescuing low paid damsels in distress - as they like to present themselves. The reality is that they can only ever deliver part of the equal pay answer and by talking down the unions they aim to create an equal pay bandwagon on the back of which they are growing very rich.

“Whatever they might say to persuade women to sign up with them, no-win-no-fee lawyers simply ride on the backs of the unions, invariably settle for the same compensation package, but then take a chunk of that compensation for their fees.

“We have seen this all round the country, thousands of female union members being persuaded to sign up to no-win no-fee lawyers and then forfeiting large chunks of their compensation as a consequence. The message is clear, stick with the union and you get full compensation and your pay sorted for the future.”