An internal email between two members of Tony Blair’s inner circle is at the centre of a BBC story which was banned from broadcast, the broadcaster has said.

Lord Goldsmith, the Attorney General, went to the High Court on Friday to obtain an injunction preventing the BBC from revealing details of the story, relating to the investigation into cash-for-honours allegations.

Lord Goldsmith said that he was acting on the request of the Metropolitan Police, which has conducted a 12-month investigation into claims that peerages were offered in return for financial support for political parties. A spokesman for the Attorney General said that police were concerned that the disclosure of information contained in the story could have harmed their inquiry. Lord Goldsmith - a member of the Cabinet - was acting independently of the Government in seeking the injunction yesterday afternoon, said the spokesman.

News of the injunction - the first to be sought or granted in the cash-for-honours affair - heightened expectations that the inquiry might at last be coming to a close.

The cash-for-honours inquiry was sparked in March last year by complaints to the Metropolitan Police by Scottish National Party and Plaid Cymru MPs that honours appeared to have been offered in return for financial support to the major parties. So far, four people have been arrested - Lord Levy, Tony Blair’s personal fundraiser, Ruth Kelly, a Downing Street aide, Sir Christopher Evans, a Labour donor, and Des Smith, a former headteacher - but there have been no charges. The Crown Prosecution Service has said there is no evidence to support a charge against Mr Smith, who told an undercover reporter that honours may be available in return for support for the Government’s City Academy programme. The Prime Minister has been interviewed twice by police, both times as a witness, not a suspect. He has not been arrested or interviewed under caution.

Jack Straw, the Commons Leader, today defended Lord Goldsmith’s right to intervene in such a politically sensitive situation. He told BBC1’s Sunday AM: “The position of the Attorney General is very well established. He is independent of Ministers; every system in the world has a figure like the Attorney General, usually called an Attorney General. When this has come up before, the Conservatives have very wisely and correctly confirmed the position as well. I cannot talk about anything else that’s happened recently in terms of the injunction.”

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