London District secretary of the RMT union said "We are concerned that any unauthorised person should have access to the tracks - particularly at that hour. There would have been time to have hidden anything
We have demanded security checks for those who work on the system but this has not been done. There are subcontractors who wander around without identification"
An LU spokesman said "The safety and security of London Underground is our top priority. Immediately an individual was found in the tunnel he was escorted from the track and questioned by the British Transport Police. The track was searched and nothing was found".