An independent “forensic” inquiry has been launched after a £25m financial black hole opened up in NHS Croydon’s accounts.

The primary care trust has asked NHS London to commission an independent review of its 2010-11 accounts.

A south west London cluster finance report said the full year “adverse variance from budget” forecast was £25.5m and that achieving break-even would mean “utilising all of the available reserves except for £2.29m of the contingency reserve”.

However, one senior figure in the area said the true size of the full year deficit could be between £30m and £50m. A cluster spokeswoman declined to comment on the claim.

The cluster is already conducting an internal review. Its spokeswoman said: “We have found that the current run rate of expenditure in Croydon indicates that the budgets were not set at the right level.”

She said hospital activity was higher than had been budgeted while quality, innovation, productivity and prevention savings and demand management plans were “not delivering as highly as we had predicted”.

The most recent QIPP report revealed Croydon’s savings target of £18m was showing a shortfall of £583,000 in month four, rising to £1.4m in month five.

The cluster’s finance report said the internal investigation “revealed a number of significant cost pressures originating from under-accruals in 2010-11. It said these included items of expenditure with no budget or insufficient level of budget and a planned year-on-year reduction in acute expenditure of 5 per cent not materialising”.

The PCT is also host to London’s specialised commissioning group and the cluster said the NHS London review would also look at this relationship.

Three London PCTs have already declared they will be in deficit by the end of the year.

Barnet, Enfield and Haringey PCTs declared last month that they were setting a deficit budget totalling £56m.

The south west London cluster said it had not asked the strategic health authority for financial support.

“Across NHS South West London it is our aim to manage this deficit within existing financial resources as far as possible,” it said.

“We have asked NHS London to commission an independent review of the 2010-11 accounts. It is important to complete a thorough and forensic review so we are sure of the facts.”

NHS Croydon has a £582m budget for 2011-12. The cluster’s latest finance report said the PCT’s planned surplus had been reduced from £8.3m to break-even. The cluster’s total surplus has been revised down by £5m to £27.2m. NHS Richmond and NHS Wandsworth have both had their year-end surplus targets raised to cover Croydon.

The PCT has pooled budgets with Croydon Council for occupational therapy and equipment service.

NHS South West London chief executive Ann Radmore and director of finance Jill Robinson signed off NHS Croydon’s accounts for 2010-11.