ACTUARIES, the highly trained professionals who calculate pensions liabilities, yesterday broke with tradition and admitted that it was too difficult to estimate how long human beings might live.

For more than 80 years the Continuous Mortality Investigation (CMI) has supplied mortality tables that showed how lifespans were lengthening and how they might continue to improve.

Yesterday, the CMI acknowledged after the release of its latest set of tables that there was so much room for error it was no longer sensible to offer a single set of predictions. It said that projections of future mortality had not been done with the latest tables “because of the uncertainty surrounding future improvements”.

Actuaries use mortality tables to work out how much money companies should put aside for future pensions obligations. Insurers use the tables to price products such as life insurance. Improvements in medical care and diet and reductions in manual work and smoking have undermined established patterns in longevity improvement, causing actuaries to be accused of misleading companies over their data.

Kevin Wesbroom, principal consultant at Hewitt Associates, the actuarial consultancy, said that the admission was not a mere technical point. “If the FTSE 100 companies assumed that people will live two years longer, their pension deficits would increase by around £20 billion,” he said.

The new tables, called the 00 Series, look at mortality rates between 1999 and 2002. The previous tables, the 92 Series, were based on death rates for 1991 to 1994. The 00 Series showed that 12.85 men of every 1,000 aged 65 would die that year; the 92 Series put the figure at 18.12, meaning mortality rates had improved in that period by 29 per cent.