Ron Noble told security officials in London he knew of no clear threat from terrorists or organised criminals.
But he said the UK did not have enough immigration staff and raised concerns about passport checks in Europe.
The warning came as police and military personnel prepare to carry out security exercises on the River Thames.
Mr Noble said Britain did a good job of screening identification documents against Interpol’s database of lost and stolen travel documents.
But he criticised other European countries for failing to check passports and said some 500 million flights were not screened last year.
Some 44 officers from the Met’s marine, firearms and air support units will join 94 Royal Marines in tests focusing on potential security threats ahead of London 2012.
‘Nothing to chance’
The military equipment includes rigid inflatables, landing craft, raiding craft, a patrol boat and a Lynx helicopter.
The officer in charge of Olympic security, Assistant Commissioner Chris Allison, said the exercises were not in response to any specific threat.
“All of our planning is designed to mitigate against potential risks during the summer of 2012,” he said.
“And this is an example of where we will be using specialist military capability to support us.”
The Olympic Games, from 27 July to 12 August, will represent the UK’s largest ever peacetime security operation.
Crime and Security Minister James Brokenshire said: “This exercise forms part of the comprehensive testing and exercise programme that is crucial in securing the Games.
“We are leaving nothing to chance to ensure we deliver a safe and secure Games.”
The Met estimates 9,000 officers will be on duty in the capital on peak days with 12,000 nationally on “Olympic duty”.
source: BBC © 2012