Private equity firm Kohlberg Kravis Roberts is understood to be considering financing its £10bn (€14.8bn) joint bid for retailer Alliance Boots with a new controversial form of debt package that offers little protection to lenders if the target company's financial position deteriorates, according to bankers.

The development comes days after the owners of the UK's largest pharmacy chain opened its books to KKR and Stefano Pessina, Alliance Boots' deputy chairman, after they raised their bid per share by 4% to £10.40 last Friday.