Four detonators in an open metal box labelled "explosives" were housed in a Tube station storage room normally used for keeping liquids, a union has said.
Tube Lines is a private maintenance company on the Underground
The Transport Salaried Staffs' Association complaints led to the boxes being moved from Golders Green London Underground station to a safer place.
Tube Lines blamed an administrative error which was quickly corrected.
The detonators are laid on tracks and explode when trains go over them to warn maintenance teams working nearby.
The Transport Salaried Staffs' Association (TSSA) added that Tube Lines - one of the two companies in the Tube public private partnership (PPP) scheme - had been criticised by the Railway Inspectorate over the matter.
The union's general secretary Gerry Doherty said it showed a fall in safety standards and called for maintenance work to be brought back into the public sector.
"This had the potential to be a very dangerous situation both for our members and the wider public," he said.
"Thankfully, Tube Lines responded quickly to our concerns and have now dealt with the situation.
"But what it does show is a worrying slip in safety standards on the world's busiest Tube line under PPP. Where we should have a gold standard, we have too often seen the second rate."
A spokesman for Tube Lines said: "When undertaking an inspection, union representatives found some train detonators in a locked room at Golders Green.
"Tube Lines acted quickly to address an administrative error regarding storage licences and to ensure train detonators are consistently stored in locked boxes as well as in locked rooms.
"Feedback from the Railway Inspectorate to Tube Lines on our response was positive."
Both TSSA and Tube Lines' representatives appeared later on Wednesday before the House of Commons Transport Committee.
Brian Harris, the regional officer for the Unite Amicus union, told the committee the Tube maintenance companies had a "disjointed management nature" and that the Tube was "lurching from one crisis to another with safety put on the back burner".