Tube drivers may refuse to work after their union said it did not get the assurances it sought in a meeting with the mayor and underground bosses.
Unions want reassurances from London underground
The Rail, Maritime and Transport (RMT) union's Bobby Law said members would not strike but the union would support any that felt too at risk to work.
He said they wanted assurances current staffing levels would be maintained.
But Ken Livingstone said the meeting was "very productive" and suggestions to improve safety had been made.
"We have had a very useful meeting and several actions will arise out of this over the days and weeks to come," the mayor said.
'Work with poise'
Mr Law said his members would be "extremely disappointed" by the outcome.
He said: "We aren't looking at industrial action as a whole, but any individuals feel unsafe to work, we will remind them that they have got the right to refuse, and do our best to support them as a union."
Mr Law said the union would be meeting with London Underground (LU) bosses again in the next few weeks.
LU's managing director Tim O'Toole said he had already spoken to drivers about their "legitimate" concerns and worked closely with British Transport Police throughout Thursday.
"We believe LU staff will continue to work with the poise and professionalism they have demonstrated consistently over the last two weeks," he said.
An RMT spokesman said earlier in the day that the issues the union had wanted to be addressed were:
- They want LU to drop plans to weaken or abolish section 12 - which came in after the King Cross fire and sets out minimum standards for safety on sub-surface trains
- The return of guards to Tube trains
- A return to dedicated gate-line staff that patrol ticket barriers
- Fully trained supervisors to accompany any contract work on the network
- Better equipment, such as breathing apparatus, for train operators and station staff
- The situation around amber alert needed to be clarified - Mr Kotz said on 7 July LU closed the entire network on amber alerts but on Thursday the network remained open