The line operator has now added an additional lock to the platform gate at Abbey Wood along with other security measures.

Howard Smith, Transport for London’s chief operating officer for the Elizabeth line, said: “Clearly a customer should not have been able to board an Elizabeth line train at Abbey Wood in November.

“Our operator, MTR Elizabeth line, has put in additional procedures to increase security and ensure this cannot happen again and we have worked with them to investigate this incident.”

The incident does not have to be reported as a serious breach of health and safety regulations, Construction News reported. 

The Elizabeth line has suffered numerous delays since it was announced in 2009 and is now due to open to passengers in June, more than three years after an initial target date of 2018.

However, the Bond Street station will not be ready by June, it was announced on Monday.

The repeated delays to the £19bn Crossrail project are a blow for the mayor of London, Sadiq Khan, who also chairs TfL.

When fully operational the line will run from Reading to the west of the capital to Shenfield in the east, becoming the newest tube route since the opening of the Jubilee line in 1979.