Police partially evacuated London Heathrow Airport's Terminal 5 this afternoon after a security alert sparked by a man with a suspect package, they said.

Passengers were unable to enter the terminal, which is British Airways' global hub at the world's busiest international passenger airport.

"There is a security alert, we're checking out a package," a Scotland Yard police spokesman told AFP.

About 40 minutes later, police confirmed the alert was over.

"It's all stood down. There was a package we were concerned about; it's all been cleared," a spokesman said.

Heathrow said the departures section of the building had been partially evacuated, with passengers unable to enter the terminal.

"Terminal 5 departures has been partially evacuated by the Metropolitan Police," a spokeswoman for the airport said.

Later, it issued a statement saying the incident was over and the terminal would be re-opened to passengers "shortly".

Terminal 5, the largest free-standing structure in Britain, cost £4.3 billion ($6.9 billion, five billion euros) to build and was 15 years in planning and construction.

It is able to handle 30 million passengers a year and opened with great fanfare in March 2008.