Britain's most senior counter-terrorism officer quit today after his security blunder forced police to bring forward a major operation to thwart a suspected al-Qaeda plot involving Pakistani nationals.
Prime Minister Gordon Brown said officers had been dealing with a "very big terrorist plot" which security officials had been following for some time.
He said he would speak to Pakistan's President Asif Ali Zardari, saying more needed to be done on tackling extremism.
Scotland Yard's Assistant Commissioner Bob Quick was photographed entering Mr Brown's 10 Downing Street residence yesterday openly carrying a secret document revealing plans to crack down on a group suspected of plotting attacks in Britain.
Hours after he and the document were pictured, police arrested 12 men in unusual daylight counter-terrorism raids across northwest England.
Most of those arrested were Pakistani nationals.
"I have today offered my resignation in the knowledge that my action could have compromised a major counter terrorism operation," Mr Quick said in a statement.
"We know that there are links between terrorists in Britain and terrorists in Pakistan," Mr Brown told Sky News.
"Pakistan has got to do more to root out the terrorist elements in its country."
The document carried by Mr Quick, headlined "Briefing Note: Operation Pathway" and marked "secret", described the operation as a "Security Service-led investigation into suspected AQ (al-Qaeda) driven attack planning within the UK".
It said 11 people were targeted for arrest, 10 of them Pakistan nationals in Britain on student visas, and one British.
Detectives are now questioning 12 suspects arrested during the raids by hundreds of officers in Liverpool, Manchester and Clitheroe, Lancashire.
The arrested men ranged from a teenager to a 41-year-old man, police said.
Reuters