Two tube trains “narrowly avoided” a head-on collision when they were diverted along the same track between Finchley Central and Mill Hill, according to a railway workers’ union.

The National Union of Rail, Maritime and Transport Workers (RMT) claims a northbound train leaving Finchley Central on Monday evening had to apply its emergency brakes when it emerged another train was heading south along the same Northern Line track from Mill Hill East.

Both trains were in service and carrying passengers, with the RMT claiming it was only the “swift actions" of the driver that avoided a “potentially fatal” crash.

London Underground admitted today the two trains had been diverted along the same track, but accused the union of scaremongering.

The RMT this week demanded a full safety investigation by the Office of Rail Regulation, a call backed by Labour’s London Assembly transport spokeswoman Val Shawcross.

The union also called on London mayor Boris Johnson to scrap plans for more driverless trains in the capital.

RMT General Secretary Bob Crow said: “Nothing exposes the lethal gamble of driverless tube operation better than the incident on the Mill Hill line on Monday night.

“RMT will not allow tube chiefs to sweep this serious incident under the carpet and we are demanding a full investigation by the regulator to establish just how two tube trains ended up routed onto a single section of track, travelling towards each other.

“This union will not allow tube safety to be compromised by a cavalier management that chooses to ignore the warnings and questions raised by our safety reps.”

London Underground’s operation director Nigel Holness played down the incident, branding the union’s approach “irresponsible”.

He said: “It is totally irresponsible of the RMT leadership to attempt to scaremonger in this way.

“There was no risk to our staff or customers, the signaller ordered the train to come to a halt as it was departing Finchley Central station and when it was around one kilometre away from Mill Hill East, and well clear of the single track section of the Mill Hill East branch.

“However, the signalling would have also prevented it travelling onto the single track. We are of course looking closely at the incident to ensure that our processes are working and being followed correctly, but we have an excellent safety record and we would never compromise the safety of our staff and customers.”