LONDON - Britain's most senior counter-terrorism officer quit on Thursday 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 Brown's 10 Downing Street residence on Wednesday 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, Quick said in a statement.

Most terrorist plots in Britain since September 11, 2001 have had links to Pakistan, including suicide bombings in July 2005 which killed 52 people on London's underground and bus network.

We know that there are links between terrorists in Britain and terrorists in Pakistan, Brown told Sky News. Pakistan has got to do more to root out the terrorist elements in its country.

SECRET DOCUMENT REVEALED

The document carried by 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.

Opposition politicians, who clashed with Quick last year after he ordered the arrest of their immigration spokesman as part of a probe into leaks of government information, said they believed the senior officer had to go.

Quick will be replaced by Assistant Commissioner John Yates, a high-profile officer who led a probe into alleged political corruption that overshadowed the final months of former Prime Minister Tony Blair's time in office.

It is unacceptable for Britain's most senior anti-terrorist officer to have had such an extraordinary lapse in judgment, said Conservative Home Affairs spokesman Chris Grayling.

To put the security of his police officers and the operation at risk has rendered his position untenable.

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