By Ana Nicolaci da Costa and Simon Challis

LONDON (Reuters) - Pension deficits at leading companies rose by 1 billion pounds in the third quarter as stock markets tumbled, but the damage could have been far worse if shares had not rebounded in September, data showed on Monday.

Consultants Mercer said both FTSE 100 and FTSE 350 companies saw their pension scheme deficits grow, to 6 billion pounds from 5 billion pounds and 9 billion pounds from 8 billion pounds, respectively.

The deficits would have been worse, however, had the reporting period not included September when stocks rebounded significantly as credit crisis fears were put on a back burner.

The FTSE 100 deficit was as high as 30 billion pounds in mid-August, as the value of equities held by pension funds lurched downwards, hit by a credit crunch triggered by the U.S. subprime mortgage crisis.

"There would have been a substantial impact on company valuations if FTSE 100 companies had reported in mid-August. The data highlights the volatility of accounting deficits," said John Hawkins, principal in Mercer's financial strategy group.

But, in overall terms, the Mercer data shows the state of pension funds is relatively strong. The deficit of 9 billion translates into a small deficit of assets when compared with overall pension liabilities of 438 billion pounds in the schemes that comprise the survey.

The aggregate funding level of the FTSE 350 pension schemes was 98 percent at the end of the third quarter, the same as at the end of the second quarter. FTSE 100 schemes had a funding level of 98 percent, down from 99 percent.

Generally favourable stock and bond markets over the past year, along with record company cash injections to address major deficits in some schemes, have helped pushed occupational pension schemes closer towards the black.