A number of London Underground drivers have been sent home for refusing to take out trains they do not believe have been properly safety-checked, a union has claimed.
In an escalation of the long-running row over plans for all-night Tube services, Aslef said seven of its members had been sent home without pay.
A union spokesman said: "Aslef will fully support our members who are acting to protect their own safety and that of the travelling public.
"Passengers will rightly be concerned by the attitude of the company, whose determination to force through changes to rosters while ignoring safety concerns is exacerbating the current dispute."
The Rail, Maritime and Transport (RMT) union said its members had been threatened with being sent home for refusing to take out trains they claim have not been properly prepared.
General secretary Mick Cash said: "Instead of addressing the issues at the heart of the night Tube dispute, LU are instead ignoring safety concerns and threatening drivers who refuse to take out trains that have not been properly prepared. That is outrageous and deliberately inflammatory."
The union said the action of refusing to take out trains was hitting services but LU was reporting only minor delays.
London Underground chief operating officer Steve Griffiths said: "We are working closely with the unions to resolve the dispute on pay and night Tube and we are committed to continuing constructive discussion.
"Safety is never compromised and the safety of our customers and staff is our top priority at all times.
"Our measures to mitigate industrial action by union members include experienced managers undertaking some of their staff's duties if required. Any suggestion that we would use inexperienced staff is completely false."
A second strike by LU workers is set to be held next week unless the dispute over pay and conditions is resolved.
The new night Tube is due to start at weekends from September 12.
Steve Griffiths, LU's chief operating officer said later: "Safety is never compromised and the safety of our customers and staff is always our top priority at all times.
"All staff preparing our trains for customer service are experienced, fully trained and qualified. The Office of Road and Rail (ORR) is also fully aware of our plans, which uses a common procedure and has been working safely and reliably for many years.
"Today, a small number of staff refused to drive trains because they were taking part in industrial action as part of the broader dispute on pay and night Tube. Given their refusal to work, and as is standard practice, we requested that they return home. There is a good service on all London Underground lines today."