Leading private equity firms are unlikely to participate in any recapitalisation of Ambac and MBIA, increasing the pressure on banks to come up with a rescue package for the troubled US bond insurers.

A number of firms, including Bain Capital, Carlyle Group, Kohlberg Kravis Roberts and TPG, have looked at investing in the cash-strapped groups, which guarantee the value of everything from municipal bonds to the most complicated mortgage securities. These investors have all concluded that the risks are too great, according to people familiar with their thinking.