The police watchdog has asked for an investigation into London's chief officer over the shooting dead of Brazilian Jean Charles de Menezes.
The de Menezes family accuse Sir Ian Blair of misleading them
Home Secretary Charles Clarke said he would reply "shortly" to the request regarding Metropolitan Police Commissioner Sir Ian Blair.
The family of Mr de Menezes say Sir Ian "misled" them and the public after the death at a Tube station on 22 July.
The Brazilian was shot by police, who thought he was a suicide bomber.
'Great step forward'
Under legislation, the Independent Police Complaints Commission (IPCC) has to ask the home secretary to appoint the senior investigator if a complaint relates to either the Commissioner, or a Deputy Commissioner, of the Met.
Patricia da Silva Armani, a cousin of Mr de Menezes, said: "This is a great step forward in our campaign for justice.
"I am so happy, we all are so happy, it's the best news."
Another of his cousins, Alex Pereira, said: "I hope this new investigation into his behaviour brings out the truth."
The de Menezes family have also lodged an official complaint about the length of time it took for police to inform them of the 27-year-old's shooting, at Stockwell station.
This will be looked at as part of the main IPCC investigation, which is expected to be concluded by the end of the year.
The report will then be sent to the Crown Prosecution Service, which will decide whether to bring charges against any of the officers involved.