The state-appointed pensions lifeboat faces an £800m bill if administrators are called in at Carillion, according to pension experts.

Carillion has 13 final salary schemes in the UK with 28,500 members and a combined pensions deficit of £587m.

But independent pensions consultant John Ralfe warned that could balloon to £1.4bn if the firm goes into administration, due to the way pension deficits for collapsed companies are calculated. That would leave the Pension Protection Fund on the hook for around £800m, Mr Ralfe estimated.

He said: “The overall hit to the PPF, before any claw back from the value of Carillion's assets, would be around £800m, making it one of the PPF's largest ever hits.”

The PPF, which answers to the Department for Work and Pensions but is funded by a levy on solvent pension schemes, had £6.1bn of cash reserves as of last March, so would be “big enough to cope” with the burden, Mr Ralfe said.