Money market funds are the “sacred cows” of the mutual fund world, boring, predictable and dependable. In the last year the total amount of money invested in money market funds has soared to almost $3,400bn as investors have fled from riskier products like equities. By regulation and design they invest only in top quality short term assets and are considered by retail investors to be as secure as a bank account.

However, the Reserve Primary Fund, the oldest in the US, on Tuesday night said that investors could lose money as a result of it holding debt in Lehman, which has filed for bankruptcy. Meanwhile, Putnam Investments’s board voted to close its $12bn Putnam Prime Money Market Fund on Wednesday, after institutional investors quickly pulled out cash.