"It might seem unnecessary for employers to check their practice here, as it's one of those accepted 'truths' that IT contractors get paid much more than IT employees. But the situation will vary between companies, so all companies must do the calculations - and not just those who have driven down their contractor rates in the last couple of years," he added.

The Public and Commercial Services (PCS) union was worried, however, that employers might try to circumvent the legislation by ensuring that contractors do not stay in the same roles for 12 weeks.

"Agency workers should only be used in limited circumstances for genuinely short-term cover [and] when they are used they should receive equal pay and conditions in return for the contribution their work brings to organisations," the union said, adding that it will be keeping a "close eye" on how the regulations work in practice.

But a spokesperson for the Department for Business, Innovation and Skills insisted: "Measures are in place to ensure that agencies and businesses don't purposefully try to get round the rules.

"If any worker feels that they are entitled to their week 12 rights, or a pattern develops where their work ends prior to this, then we would encourage them to speak to their agency in the first instance and then [arbitration service] Acas if the issues are not resolved."