Leading banks were on Tuesday night under renewed pressure to fast-track plans to shore up struggling bond insurers after Warren Buffett revealed he had offered to take over the high-quality portions of the insurers’ portfolios.

Monoline meltdown?

View our interactive guide on how the ratings decisions affect monolines

The billionaire investor told CNBC he had offered to take more than $800bn of municipal bonds guaranteed by three bond insurers – Ambac, MBIA and FGIC. They are fighting to retain the triple-A credit ratings on which their business model depends amid huge subprime losses on their non-municipal debt portfolios.