Investors’ fears of prolonged financial turmoil deepened on Thursday as blue-chip downgrades sent European and US shares into a tailspin and oil prices jumped above $140 a barrel for the first time.

The sell-off underlined the markets’ gloomy view of the financial sector amid predictions that banks and brokerages would suffer more large writedowns and be forced to cut dividends and raise capital again. Concerns over the health of the financial sector were underscored on Thursday by news that the US Federal Reserve was considering relaxing rules governing investments into banks by private equity firms to encourage capital-raising.