The 'secret' documents clutched by Mr Quick were clearly on show
Britain's most senior counter-terrorism officer has been strongly criticised over his role in "exposing" a highly secret police operation.
Police brought forward raids after Assistant Commissioner Bob Quick revealed secret papers to photographers when arriving for a briefing at No 10.
Twelve men remain in custody after anti-terror officers swooped in Manchester, Liverpool and Lancashire.
Opposition MPs say Mr Quick's judgement has been called into question.
The officer has apologised to Metropolitan Police Commissioner Sir Paul Stephenson for the error.
Home Secretary Jacqui Smith made no comment about Mr Quick's mistake, saying the decision to act had been an "operational matter for the police and the security service".
'Lack of judgement'
Shadow home secretary Chris Grayling told BBC Breakfast the way the officer had handled the papers amounted to a "catastrophic blunder".
It displayed a "huge lack of judgement by the man in charge of our counter-terrorism operations" and raised "big question marks over his ability to do the job", he said.
"This could have blown a major counter-terrorism operation - very fortunately it didn't, the arrests were still made.
"We don't know yet whether the arrests were accelerated and whether that will affect the investigation.
"This was an incredibly potentially damaging thing to do and to have it happen at the hands of Britain's top counter-terrorist officer is an extraordinary situation."
He called on the police and the government to explain what steps would be taken to ensure such a situation did not happen again.
Liberal Democrat home affairs spokesman Chris Huhne said: "Assistant Commissioner Bob Quick seems to be increasingly accident prone which is potentially dangerous given the serious responsibilities of his role."
Charles Shoebridge, a former counter-terrorism intelligence officer, told BBC Radio 4's Today programme the act of revealing the memo should be regarded as "quite grave".
"Without any question it is a serious blunder," he said, but added that it was easy to understand how such a mistake happened when officers were moving between meetings.
The most important question was whether it had jeopardised any part of the anti-terror operation, he said.
Series of raids
Mr Quick has remained a controversial figure since ordering the arrest of senior Tory MP Damian Green over alleged security leaks last year.
In December, he apologised after accusing the Conservatives of involvement in newspaper stories about his wife's hire car business.
However, former London mayor Ken Livingstone said al-Qaeda would be "delighted" if someone with Mr Quick's experience had to step down "for one mistake of holding a piece of paper the wrong way".
The document, clearly marked "secret", carried an outline briefing on an on-going counter-terrorism operation.
It contained the names of several senior officers, locations and details about the nature of the overseas threat.
Some hours after the Downing Street incident, hundreds of officers from the North West counter-terrorism unit carried out a series of raids.
Jacqui Smith praises officers over the terror raids
Armed officers arrested one man outside the main library at Liverpool John Moores University.
Police sealed off another three premises in the city for searches, along with five addresses in the Cheetham Hill area of Manchester, including an internet cafe, and a guest house in Clitheroe, Lancashire.
Two of the suspects are understood to have been arrested at a Homebase DIY store in Clitheroe, where dozens of police officers carried out a raid.
Ten of those arrested are Pakistan-born nationals on student visas and one is a UK-born British national.
Their ages are not known but range from a youth in his mid-to-late teens to a 41-year-old man.
Mr Quick had been attending a Downing Street meeting in his role as lead for counter-terrorism and for the Association of Chief Police Officers (Acpo).
Details of the information revealed cannot be reported.
A Scotland Yard spokesman said: "Assistant Commissioner Quick accepts he made a mistake on leaving a sensitive document on open view and deeply regrets it."
Footage of the anti-terror raid at Liverpool John Moores University
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.