A major rail accident was narrowly avoided today after two construction drills burst through the wall of a train tunnel.

First Capital Connect (FCC) cancelled all Northern City Line services between Finsbury Park and Moorgate from 10am this morning after the incident which happened just north of Old Street station.

The alarm was initially raised after the driver of the Welwyn Garden City to Moorgate service with passengers on board saw muddy water pouring on to the roof of his train, and services were suspended.

A second empty train was sent along the line just after 11am to investigate the problem but as it approached the area at low speed, two large piling drills came through the side of the tunnel.

The train stopped in time but the driver was shaken, said FCC.

British Transport Police has visited an office construction site above the line and the Rail Accident Investigation Branch has been notified.

The train company has this afternoon praised its drivers for their vigilance.

FCC managing Ddirector Neal Lawson said: “This is a serious incident that could have ended very differently had it not been for the vigilance and prompt reporting and actions of our drivers.

“We carry two million people a year on the Northern City Line and whoever is responsible for this must be held to account.

“Meanwhile we have published details of alternative travel arrangements and are urging our passengers to check www.firstcapitalconnect.co.uk for the latest news but the line will be out of action for the rest of the day at least.”

Network Rail said specialist tunnelling contractors had been sent to the scene to assess the damage.

Phil Verster, Network Rail route managing director, said: “Our engineers are on site assessing the damage and we will restore the railway as soon as possible but unfortunately the line will remain closed for the rest of the day.

“Passengers should check with National Rail Enquiries or First Capital Connect for the latest information.”

During tonight’s peak, FCC services will be running from into and out of King’s Cross or Finsbury Park instead of Moorgate.

Customers are advised to use London buses or the Tube (Victoria line or Piccadilly line) to reach Finsbury Park for connecting services. Tickets will also be accepted on Greater Anglia services from Liverpool Street station to Cambridge, Enfield Town and Hertford East.

The line is normally closed at the weekend and engineers will be working to reopen it in time for Monday morning’s service.