The FTSE 100 could see its combined balance sheet worsen by £100bn as a result of stricter accounting rules for pension schemes and growing pressure to increase contributions.
That is according to a report published today by actuarial consultants Lane Clark and Peacock (LCP), which warns that more than a quarter of FTSE 100 firms could take a £1bn hit.
This would arise if the International Accounting Standards Board (IASB) updates IFRIC 14 rules so that firms are forced to show agreed deficit contributions on balance sheets.
The changes could be put forward for consultation in the coming year after being shelved in 2015, and hit balance sheets by far more than the £13bn recorded last year.
"The new proposals are intended to be more 'principles based' to better reflect the reality of the position rather than legal and accounting technicalities," the report states.
"Depending on what the new rules say, they could affect most or all UK companies, rather than just a handful as at present."
Pension liabilities reported under the IAS 19 accounting standard are usually less than the more prudent funding liability agreed with trustees to set deficit contributions.
IFRIC 14 rules currently mean that some companies recognise an extra liability on the balance sheet, but this depends on a "legal lottery" based on their pension scheme rules.
Although more firms could be impacted, LPC said that changes to IFRIC 14 remain "a little way off", giving companies time to put in place strategies to manage the risk.
This could include contingent asset arrangements such as escrow, asset-backed funding, surety bonds and letters of credit to give trustees the security they need.
"Over 10% of FTSE 100 companies with defined benefit pension schemes disclosed they already use these arrangements," the report states.
"We expect this figure to grow as companies adopt new ways to support their pension commitments in the future."
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