LU is investigating the incident

The Iraqi national was found in a "bolt hole" - an area used by maintenance staff to stand back from moving trains.

He was found in a tunnel between Seven Sisters and Finsbury Park on the Victoria Line at 0300 BST on 20 April, but details have only just emerged.

The tracks were searched and the man was released without charge after being questioned by police.

A spokesman for London Underground (LU) said the man was seeking shelter and managed to get onto the tracks after staying on a platform until the station closed.

We work very closely with the intelligence and security services LU spokesman

But Bobby Law, London regional organiser of the Rail, Maritime and Transport union, (RMT) said the case highlighted concerns over safety on the Tube at a time of police warnings of the threat of terrorist attacks in London.

"This incident highlights our concerns about the lack of security on the Tube," he said.

"We are concerned that any unauthorised person should have access to the track, particularly in the early hours of the morning."

Safety priority

"When we raised this with the company, we were told it was a not a problem."

A London Underground spokesman said: "The safety and security of London Underground is our top priority.

"We work very closely with the intelligence and security services.

"All staff, including contractors and sub-contractors are checked and no-one working for LU can enter a station without signing in.