Pension trustees are insisting that any bidder must pledge to fill the gap in the pension pot before a deal can be struck to buy WH Smith.

Private equity firm Permira is the only bidder so far, tabling a �940m takeover offer for the retailer.

Shareholders fear the company's �200m pensions shortfall could scupper a deal.

Up front

Backed by finance from banks, Permira has promised to at least match the current funding programme to make good any shortfall over 10 years.

However, the trustees want to speed up the pension repayment to protect pensioners in the event of bankruptcy.

The former chairman of WH Smith, Martin Taylor, now heads the board of pension trustees.

He told the BBC Radio 4's Today programme that WH Smith was making the stipulation to protect the pension fund only to "bidders who want to put an awful lot of borrowing into the company".

"We'd like to make sure the pensioners are no worse off in terms of the risk they bear. The obvious way to do that is to put all the money up front," Mr Taylor said.