Sir Ian Blair, the head of the Metropolitan Police, is to be personally investigated for his conduct and statements immediately after the killing of Brazilian Jean Charles de Menezes in South London in July.

The investigation follows a formal complaint by Mr de Menezes's family to the Independent Police Complaints Commission and under a piece of legislation never been used before, the investigating officer had to be formally approved by Charles Clarke, the Home Secretary.

Mr de Menezes, a 27-year-old electrician, was shot seven times in the head after being mistaken for a suicide bomber by officers at Stockwell Tube station on July 22.

The circumstances of his death are already the subject of a major IPCC investigation, due to be completed by the end of the year. But the watchdog announced today that it was to launch a separate probe into the conduct of the Commissioner and some of his colleagues.

The Jean Charles de Menezes Family Campaign tonight said it was delighted at the news of the IPCC inquiry. Alez Pereira, cousin of Jean, said: "This is fantastic news and a great victory for our campaign."

The Menezes Family Campaign said in a statement that: "Sir Ian Blair, as head of the Metropolitan Police, bears ultimate responsibility for the Menezes killing and that he deliberately tried to cover up what really happened after Jean’s death."

Patricia da Silva Armarni, Jean’s cousin who shared a flat with him in London, said: "The pain of the loss of our cousin at the hands of the police was immense. But the fact that the police lied about what happened and tried to cover up the truth made our pain unbearable."

The statement claimed that police officers would have known within minutes that they had killed an innocent man, "but Sir Ian claimed he did not know for 24 hours Jean was innocent - that is unbelievable".

The family campaign said that it believed off the record statements were given by senior Met Police figures to journalists in the immediate aftermath of Jean’s killing, suggesting that Jean was acting suspiciously and was linked to the terrorist operation.

A family campaign spokesman said: "We believe these ’off the record’ briefings were attempts to mislead the public and cover up the fact that the police had killed an innocent man. It is inconceivable that Sir Ian Blair and other senior police officers were not aware that these briefings were taking place, as they were reported widely in the media.

"As head of the Metropolitan Police, if Ian Blair did not know about these briefings then he is incompetent, if he did know, he is implicated in them."

The family's letter of complaint, sent last month, alleged that Sir Ian and other individuals misled the public on the day of the killing. Sir Ian told a news conference that day that he believed the dead man had been directly connected to the failed suicide bombings of July 21.

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