Although the company’s board has backed the deal, the trustees are at loggerheads with KKR over how much the new owners should pump in to the fund, which has 66,000 members.

The trustees are concerned that KKR, which is working in partnership with Alliance Boots’ deputy chairman and biggest shareholder Stefano Pessina, is funding the deal through £8billion of borrowings. It has asked the buyers to plug the fund’s £305million deficit and make total contributions of £1billion.

KKR is understood to have offered to make cash injections of £340million over a number of years and to provide a £600million security package to protect the fund should the company go bust.

The trustees are considering taking legal action unless the differences are resolved by the time the deal is due for approval at a court hearing on June 21.