Unite says all installations within a 5 mile radius of the drifting gas must be fully evacuated and powered down.
Regional Officer for Offshore Workers Willie Wallace said: “We have more than 15,000 members working offshore day in day out in highly dangerous conditions and their safety at work is paramount to us.
“This incident cannot be underestimated in its seriousness and there is still a clear and present danger we believe to many of our members while the drifting gas issue continues.
“Yesterday, there were still 80 workers remaining on the nearby Shearwater installation. The risk may be low but our concern is that if the drifting gas was to hit any of the neighbouring installations, the results could be catastrophic.
“Health and safety should never be diluted due to commercial pressures and any risk at all is too high a risk for offshore workers and their families.”
RMT has also expressed relief at the safe and successful evacuations so far.
RMT general secretary Bob Crow said: “There is still a free flow of gas into the atmosphere and that remains to be stopped.
“It took only a couple of hours to get most people off and the last 19 were evacuated by boat because it was too risky to continue using helicopters.”
RMT offshore organiser Jake Molloy said: “Total acted very swiftly in getting everyone off, but the potential still exists for catastrophic devastation.
“If the gas cloud somehow finds an ignition source we could be looking at complete destruction.
“This is an unprecedented situation and we really are in the realms of the unknown, but the urgent need now is to find a way of stopping the flow of gas.”