A runaway engineering train caused havoc on the Tube today and is due to be investigated.

London Underground is investigating after a runaway maintenance train travelled four miles on the Tube this morning, causing massive disruption.

The engineering train became decoupled while it was being towed on the Northern Line near Archway as part of Tube Lines’ engineering work.

The train then carried on travelling until it got to Warren Street and stopped. The Charing Cross branch of the Northern Line was then shut down all morning and commuters suffered huge delays.

RMT assistant general secretary Pat Sikorski said: “RMT is appalled and horrified at the major incident that occurred this morning during passenger traffic hours and which could have very easily resulted in disaster.

“The runaway train, which it is suspected broke loose from a failed emergency coupling as a broken down grinder unit was being pulled northwards at 6.40 am this morning, represents a safety failure of the highest order.

“We understand that a collision with a passenger service train leaving Archway was only narrowly avoided.

“The overnight works were the responsibility of TfL subsidiary Tube Lines, managed by the private company Amey, and RMT are seeking urgent answers as to whether or not third party contractors were involved in this incident – particularly as the findings from Potters Bar ruled that third party contractors should not be involved in rail maintenance works.

“This horrific failure comes against a background of a systematic reduction in safety-critical jobs and safety procedures as a direct result of TfL’s financial cuts programme. In the light of this morning’s events it is essential that TfL call a halt to their cuts plans and bring an end to the dilution of tube safety.”