There was an escalation in the long-running dispute over night Tube services today, with a claim drivers were sent home for refusing to take out trains they did not believe had been properly safety-checked.

The Aslef union said seven of its members had been sent home without pay.

It is reported this was the reason for disruption on the District line earlier.

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."

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 Tube workers is set to be held from Wednesday next week unless the dispute over pay and conditions is resolved.

The new night Tube is due to start at weekends from September 12, though this date is looking increasingly unlikely.