Andrew Cuomo, the New York attorney-general, on Monday sued Charles Schwab, the biggest independent broker in the US, over its sales and marketing of auction-rate securities, in the latest stage of his investigation into the collapse of the market.

Schwab, which was threatened with a lawsuit by Mr Cuomo last month, has vowed to fight any civil fraud charges, saying the “real culprits” were the Wall Street firms that created, marketed and then abandoned the market.