Fresh security concerns have hit Heathrow Terminal 5 after a Home Office raid found an illegal immigrant on the site.

It is believed that the worker, who was from Russia, was working for painting firm S Lucas, a subcontractor to Mace.

The discovery was made after a Home Office audit of the £4.3bn project, which is reputed to have the tightest security checks of any UK construction scheme.

The incident has fuelled concerns over security and worker vetting on the UK’s major projects, particularly over fears that a time bomb could be installed into a building’s fabric.

A Home Office spokesperson said: “The government has made it clear that it will take a robust approach to removing people from the country where they have no legal right to be here.”

We seek, where possible, to remove from the UK any individual found to be knowingly working without leave to remain or working in breach of their conditions of entry. This includes illegal workers.