The incidents happened on the Picadilly Line through Islington

A signal failure at the height of rush-hour hit the Northern Line this morning, causing serious delays.

The fault struck as commuters were heading into work at 8.10am.

There were also minor delays on the Underground’s Metropolitan line in the morning peak - due to a signal failure at Finchley Road.

Bob Crow, general secretary of the RMT transport union, said: “This is one of the worst Tube breakdowns this year and, just weeks from the Olympics, shows that the cuts to maintenance driven by TfL and (London Mayor) Boris Johnson have left services on a wing and a prayer with no plan B.”

The Northern Line will be required to cope with a huge influx of passengers during the Olympic Games, in three weeks time.