Pension deficits borne by some of the UK's largest companies could begin to affect their credit ratings, global credit agency Moody has warned.

According to its report, Pension Deficits: Back on the Agenda, falling asset prices could place pressure on the sponsoring employers of some of Britain's biggest funds, which may lead to credit downgrades.

The credit agency said shortfall in a pension scheme's assets diverts cash that would otherwise be available for other corporate purposes, including the management of debt. It estimates that companies could see the value of their pension funds fall by between 15% and 20% during 2008.

Moody's survey is based on the top 20 companies ranked by size of the pension scheme, and includes BP, Shell, Marks and Spencer, Tesco and Unilever. A Shell spokesman said its UK scheme remains in surplus according to latest figures released on 12 December and added that falling equity markets in 2008 will have an impact on all pensions funds.