A union and Transport for London (TfL) have disagreed over whether fire alarms failed at Euston Tube station as smoke was seen coming from an escalator.
The Rail, Maritime and Transport (RMT) union said no alarms sounded on 6 July and only the "vigilance and experience" of an employee averted what could have been "a major disaster".
TfL insisted the alarms were working at the time and were retested later.
The station was closed for almost an hour because of the security alert.
A member of staff at the station spotted smoke at the top of the escalator and activated an alarm nearby.
TfL said its alarm system was automated and would have sounded within two minutes of smoke being detected, but the employee took action before this period had elapsed.
Call for inquiry
The RMT, which is currently balloting on a row over job cuts on the London Underground, said the matter highlighted its concerns about safety.
Its general secretary, Bob Crow, said: "Anyone who is in any doubt as to why we are balloting for action over the threat to safety-critical Tube station jobs only needs to look at the incident report into the Euston escalator fire four weeks ago.
"Fire-detection systems failed and it was only the vigilance and experience of station-based staff that averted what could have been a major disaster."
The union has referred the case to the Office of Rail Regulation watchdog.
TfL said the safety of passengers and staff was its "top priority".
A statement from TfL said: "This incident demonstrates the importance we place on having our staff on hand in ticket halls and platform areas to help customers and safeguard their welfare."
It said London Fire Brigade gave permission to reopen most of the station within an hour.
"If they had any serious and ongoing concerns about our fire safety equipment, they would not have done so," a TfL spokesman added.