Employment law experts said there are no enforcement measures in the law passed before the election if companies flout the rules

Legislation that the prime minister, David Cameron, boasted will protect workers on zero-hours contracts has been described as toothless by employment lawyers.

A provision in the Small Business, Enterprise and Employment Act 2015, which was passed under the Liberal-Conservative coalition government and came into force on Tuesday, intended to ban clauses that allow employers to block zero-hours employees from holding jobs elsewhere.

The new measure against so-called exclusivity clauses was much touted by Cameron in the Conservative party election campaign. He promised: “In a Britain that everyone is proud to call home, people are employed, they are not used. Those exclusive zero-hours contracts that left people unable to build decent lives for themselves – we will scrap them.”

However, lawyers say that the ban is not backed by any enforcement measures, meaning zero-hours workers have no way of taking action against employers who break its terms.

“This is utterly toothless,” said Elizabeth George, an employment lawyer at Leigh Day, the firm that is representing zero-hours workers at Sports Direct who claim they were excluded from its bonus scheme.

She said: “This piece of legislation says that employers can’t enforce these clauses in a zero-hours contract. But if they operate a policy of reducing or not offering hours to those who have work elsewhere there’s no right for workers to do anything about it.”

David Martyn, a lawyer at Thompsons Solicitors, which is acting for former workers at USC, said: “At the moment this legislation is the very definition of toothless ... there is no way this can benefit zero-hours workers.”

He said employers were not obliged to provide regular work for those on zero-hours contracts. Therefore, as soon as a staff member lets their manager know they had work elsewhere, they could find their hours at the first company reduced or cut altogether.

The Unite union said the focus on trying to ensure workers could take on more than one zero-hours job did nothing to make their employment more secure. Such contracts have been criticised by Unite and other unions because they do not guarantee a minimum number of working hours a week and and leave workers vulnerable to missing out on annual leave and sick pay.

Steve Turner, Unite’s assistant general secretary, said: “Banning exclusivity clauses is a joke. It misses the key point that zero hours confer fear and misery of those forced into them – no security, no protection and little dignity.

“With millions now declared ‘self employed’, underemployed and insecure at work, we believe the scale of workplace insecurity is vastly underestimated by the Tory government and needs to be addressed. When business fails to create decent jobs, there are serious, deep social and economic consequences for our country.”

Lawyers said companies could easily sidestep the new law. Kirsty Ayre, from the law firm Irwin Mitchell, said: “Employers can still include clauses requiring workers on such contracts to be available for work if required, which may have a similar impact to an exclusivity clause.”

Employment law solicitor Philip Landau said: “The flexible nature of zero-hour contracts means most employers won’t be unduly troubled by the new law.”

A spokesperson for the Department for Business, Innovation and Skills said: “The government is serious about tackling exploitative zero-hours contracts and we have taken the first step by banning them.

“In commencing this provision, we are able to use secondary legislation to create a route of redress for any individual whose employer ignores the ban.”

• This article was amended on 27 May 2015. An earlier version said zero hours contracts failed to provide for annual leave and sick pay. That has been amended to say such contracts leave workers vulnerable to missing out on annual leave and sick pay.