The inquest into the death of Jean Charles de Menezes will be the "most testing" inquiry the Metropolitan Police has faced for years, Sir Ian Blair has told his officers.

In a message posted on Scotland Yard's staff website, the Metropolitan Police Commissioner said his force was facing "a difficult couple of months and we must brace ourselves for criticism".

Sir Ian's message was leaked to the Justice4Jean campaign which marked the opening of the inquest today by staging a small protest outside the court.

A banner reading "Inquest not cover-up" was erected on flats near The Oval cricket ground, south London, where Southwark Coroner's Court is sitting for three months.

The hearing is being held in the Sir John Major conference room where the process of selecting a jury of 11 began this morning.

Mr de Menezes, 27, was shot by armed police less than a mile away at Stockwell Tube station after he was mistaken for a suicide bomber on July 22, 2005.

He had been followed by police surveillance teams from an address linked to one of four men who had tried to detonate bombs on the London transport system the previous day.

In his message, Sir Ian told his force that they had to accept that "Jean Charles de Menezes was an entirely innocent man and the Metropolitan Police takes full responsibility for his death".

The Commissioner said the Met could expect to face criticism as fierce as that which raged around the bungled investigation into the murder of the black teenager Stephen Lawrence and the inquiry that followed.

He wrote: "There can be little doubt that this inquest will be the most testing tribunal the Met has faced since the Macpherson Inquiry. Something in the region of 65 serving or former officers have been called to give evidence. There will be intense media coverage."

Counsel for the force would be telling the inquest that the death of Mr de Menezes happened "in the middle of perhaps the most testing operational challenge the Service has faced since the Second World War".

Nevertheless, the shooting had "robbed a family of their son" and the Met had to account for its actions at the inquest.

Sir Ian added: "Our approach will be one of humility. On 22nd July 2005, we confidently believed that our systems of command, of surveillance and of firearms intervention were among the best in the world. However, they failed in response to a previously unforeseen circumstance, suicide bombers on the run...

"No one set out that day with any intent to let a young man die. The officers involved will be giving evidence of doing what they did with good intention. They have my support and should have yours."

Sir Ian added that he was "acutely conscious" of the fact that the inquest came in the midst of a difficult period for the Met - including allegations of race discrimination made by a senior officer and the suspension of another high-ranking Asian officer.

Times Recommends

• What makes the perfect mum?
• To keep me stress-free, no more awareness days, please
• What do lottery winners buy?