THE NHS is facing a bill for backdated wages worth millions of pounds after it emerged that staff have been cheated out of the correct rates of holiday pay over several years.

Nurses and other healthcare workers are entitled to be paid "as at work" when they are on leave, meaning that as well as basic pay rates, wages should be topped up with extra payments they usually receive, such as shift allowances for working anti-social hours.

The rule changes were introduced in October 2008 but, despite the Scottish Government writing to health boards that year to advise them of the overhauled policy, it has not been implemented in some areas, leaving employees out of pocket and unions threatening legal action.

Reviews are ongoing across Scotland, with backdated payments expected to leave the NHS with a huge bill that it have been warned will run into tens of millions of pounds at a time when some health boards are already struggling to balance the books.

In NHS Lanarkshire, a review found there had been "no consistent approach" to how nursing staff were paid while on leave since 2008, with the result being that many staff had been underpaid.

So far, a review into nurses' pay has established that 1,498 are owed a combined £1.5 million.

Reviews of a further 3,300 cases are set to be carried out this year in Lanarkshire, meaning that if the results of the first reviews are repeated, the health board will have to make payouts of almost £5m to staff.

NHS Fife has already paid more than £450,000 to 1,000 nursing and midwifery staff, while NHS Ayrshire and Arran, NHS Grampian and NHS Tayside confirmed reviews were ongoing.

Scotland's largest health board, NHS Greater Glasgow and Clyde, said it had identified some workers who had been incorrectly paid and that they would be reimbursed. According to Unison, £2m has already been paid out to 3,000 Glasgow staff members, with more to follow.

NHS Lothian, the country's second-largest board, said it had identified "a small number of areas" where staff may have been paid incorrectly and that discussions were ongoing.

While health board reviews so far focussed on nurses, Tom Waterson, chair of Unison Scotland's Health Committee, said that all NHS staff, other than doctors and senior managers, were covered by terms of the Agenda for Change pay scheme.

"Health boards are aware that they should have been making these payments since 2008 but senior managers have withheld money to alleviate issues with finance," Mr Waterson said. "In other cases it hasn't been paid due to incompetence. But they have known this was coming and now we're talking about a fortune."

NHS Highland refused to provide information following a Freedom of Information request, while NHS Borders said it had complied with rules around annual leave pay. Several smaller boards, NHS Dumfries and Galloway, NHS Forth Valley, NHS Shetland, NHS Western Isles and NHS Orkney said they had not received formal claims.