Five men have been arrested during a raid on a suspected illegal passport factory in north London.
UK passports are generally considered difficult to reproduce
Officers recovered 1,800 alleged counterfeit passports, valued at £1m, from two wardrobes in a two-bedroom flat in Poplar Grove, Barnet.
The Metropolitan Police said it believed the haul was the largest seizure of fake passports in the UK.
The men arrested during the operation were from Albania, Bulgaria and Kosovo, said a police spokesman.
When officers entered the property, a counterfeit driving licence was being printed at the time from a printer in the back bedroom, police said.
The seized haul included 200 UK passports, which, according to police, are often considered by counterfeiters to be too difficult to reproduce.
The Identity and Passport Service said the fake passports were of a design which has not been used since 2001.
A spokesman said recent versions of the British passport were "remarkably" resilient to forgery.
But he added: "However, today's raids underline why we constantly have to invest in improved security devices in the document in order to keep a step ahead of the forgers."
The latest biometric e-passports have added security features such as a secure chip holding the carrier's facial details.
Also recovered in the raid was passport making equipment, such as laminates, and blank cards, commonly used to imitate driving licences.
Det Insp Nick Downing said: "This is the largest amount of counterfeit passports...the MPS [has] ever had and potentially the largest within the UK."
He said the raid, conducted under the Met's Operation Maxim unit, has disrupted a "well established and organised criminal network".
"These counterfeit passports have the potential to be sold on to anyone wanting to carry out criminal activity or illegally enter the UK," he said.
"They are sold on recklessly by those producing them without any regard to who they might be or why they might wish to avoid detection."
Those arrested, a Kosovan, three Bulgarians and an Albanian, are in custody a police stations across London.