Brexit panic knocks pound to seven-year low

Sterling suffers biggest one-day fall since David Cameron became PM after Boris Johnson backs campaign to quit EU

The pound tumbled to a seven-year low and the UK was warned its credit rating was at risk on Monday as the effect of Boris Johnson's backing for the Brexit campaign was felt in financial markets.

However, as traders and city economists wagered that the London mayor's intervention had raised the probability of a leave vote in June's EU referendum, high-profile business figures threw their support behind prime minister David Cameron's push to stay in the EU.

Related: Top firms back pro-EU letter, but supermarkets refuse to sign

Related: Why is the pound falling and what are the implications for Britain?

Related: Moody's warns Brexit would risk UK's credit rating