Security at the London 2012 Olympic Games has not been compromised, the Metropolitan Police has insisted after one of its senior officers left secret documents on a train.

In the latest Olympics security bungle, "restricted" files detailing security arrangements, minutes from senior counter-terrorism meetings, and names and contact information for police staff were left in a bag on a commuter train by a chief inspector at Scotland Yard.

A vigilant passenger spotted the documents and handed them to the Sun newspaper.

"We do not believe that the bag contained operationally sensitive documents," said a Met spokesman.

"The documents are now back in police possession. Obviously the loss of restricted material is a matter for concern but we are satisfied that this does not compromise our security operation for the Olympics."

This latest gaffe will add to security fears around the London 2012 Games, a high-profile target for terrorists, after sniffer dogs failed to detect a bomb smuggled into the Olympic Park by undercover police in a test run.

About £530m is being spent on security at London 2012, which will include 7,500 military personnel supporting 16,200 security guards inside the Olympic park, with 12,000 police officers and 6,000 more armed service personnel on the streets outside.

US government officials are reportedly unhappy with security arrangements for London 2012, and will send 1,000 American commandos and 500 FBI agents to guard the country's athletes and diplomats.

"Ensuring a safe and secure games is one the biggest challenges for the 2012 Olympics," said shadow Olympics minister Tessa Jowell.

"We can be confident in the leadership of our security planning.

"The purpose of test events and scenario planning is to ensure that security at the games is reliable and proportionate for both the public and the athletes and I am confident that will be the case."