Credit rating agencies come under the spotlight again this week as the House of Commons Treasury Select Committee begins its own inquiry.

Downgrades of sovereign ratings around Europe have provoked angry reactions and there is an ongoing police investigation in Italy.

But the former chancellor Alistair Darling has told the BBC he found a threatened downgrade on his watch "helpful".

So are the agencies villains or are they simply telling the truth?

Chief economics correspondent Hugh Pym investigates for BBC Radio 4's File on 4.

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