Immigration officers swooped following a tip-off and staged an early morning raid on the site.

The agency team visited subcontractor Bowercross Construction Limited (BCL) on the St Thomas Street site.

A Home Office Immigration Enforcement spokesman said main contractor Costain “co-operated fully with this visit.”

Three men from India were arrested at the site during the raid late last month.

Two, both aged 27, had overstayed their visas, while a 38-year-old had entered the country illegally.

Officers then arrested a 21-year-old man who was found to be working in breach of his visa conditions at an address in Church Street, Stratford.

One of the 27-year-old men and the 38-year-old were transferred to immigration detention pending their removal from the UK.

The other two men have been placed on immigration bail and must report to the Home Office while arrangements are made to remove them.

The agency said: “BCL was served a notice warning the employers that a civil penalty of up to £10,000 per illegal worker found at the site will be imposed unless proof is provided that the correct right-to-work checks were carried out.

“This is a potential fine of up to £30,000.”

The notice was given to a Costain employee to pass on to BCL.

Claire Shacklock, from the Home Office Immigration Enforcement team in East London, said: “We regularly carry out operations like this in London and have more planned in the future.

“Those we find working illegally face arrest and removal from the UK.

“Illegal working has a serious impact on communities, undermining legitimate businesses and taking jobs from those who are genuinely allowed to work.”

Costain started main site work on the London Bridge contract earlier this year after winning the deal in late 2011.

The station will remain fully operational to passengers throughout the five-year contract which is a key part of the Thameslink programme.

The site boasts a state-of-the-art biometric access control system using 15 computer-controlled access points and 18 CCTV cameras.

Costain and BCL have both been approached for comment.