A two-mile exclusion zone has been set up around the North Sea oil platform which was evacuated after a major gas leak.
Aberdeen coastguard have ordered aircraft and shipping to stay away from the Elgin PUQ platform and Rowan Viking drilling rig, about 150 miles off the coast of Aberdeen.
A gas cloud has reportedly encircled the platform and can be seen from miles away.
Operator Total E&P UK says it could take up to six months to build a relief well to help stem the leak.
Jake Molloy of the RMT union warned that the company faces "something on the scale of Piper Alpha" if the gas ignites.
The French oil giant evacuated all 328 workers from the platform after the leak was detected at 12.15pm on Sunday.
On Monday oil giant Shell evacuated 85 non-essential workers from a nearby platform, four nautical miles (4.6 miles) from the Elgin platform, because of drifting gas.
The company removed 52 personnel from its Shearwater platform and 33 from the Noble Hans Deul rig.
Drilling operations on the rig have been suspended and the wells left in a safe state. More than 100 workers remain at the Shell facility.
The Unite union has called for all oil service personnel within five miles of the leak to be evacuated.
An Aberdeen Coastguard spokesman said: "The exclusion zone was set up at around 11am on Monday.
"It is a two-mile zone for aircraft and shipping.
"The zone has been set up for safety reasons. It is a precautionary measure."
Mr Molloy said there was the potential for a "major event" if the freeflowing gas ignites.
He said: "You're looking at something on the scale of Piper Alpha here. We're looking at a potentially major event if the gas ignites.
"On the positive side, nobody's there. So the human side has been dealt with. But the potential remains for an ignition source and for the complete destruction of that installation.
"The evacuation was text book. It was done very quickly, very rapidly.
"Depending on how and if they contain this, and how long it takes, yes there are ramifications for the supply of gas. We're fortunate it didn't happen coming into winter, because that would have been a serious potential shortage of gas."
David Hainsworth, health, safety and environment manager for Total, admitted that the release could go on "for a significant period of time".
He estimated that the gas could be leaking at a rate of two kilogrammes per second, though he said there was a "large range of uncertainty" over that figure.
He said: "The gas is flammable but the platform power was turned off to minimise risk of ignition, but clearly there is a risk.
"We have taken away a series of risks but there is always a possibility, it's low but you never say never.
"The best-case scenario is that the gas in this area is not very productive and it dies off in the coming days and weeks.
"At the moment there is no real evolution of the sheen on the sea but if that was to change - and it's monitored on a daily basis - then the exclusion zone may be increased, but at the moment it will probably stay the same."
Willie Wallace, Unite union regional officer for offshore workers called for all installations within a five-mile radius of the drifting gas to be fully evacuated and powered down.
He said: "Unite the union have over 15,000 members working offshore day in day out in highly dangerous conditions and their health and 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.
"While we welcome the speedy evacuation of the Elgin and the fact that two further installations in the immediate vicinity have been down manned of all non essential staff, we are concerned that only partial evacuation has taken place on the other installations in the area so far."
The operation to discover and plug the source of the leak continues on Tuesday.
A Total spokesman said: "Our main priority today is working out how the incident occurred, the extent of it and how we can control and minimise any impact on the environment.
"We have a dedicated team working 24 hours to address the issue."
The company has flown in experts from as far afield as Texas who are working on stopping the leak.
Two aerial surveillance flights were carried out on Monday to examine a sheen on the water near the platform. Another two flights are expected to be carried out on Tuesday.
The sheen, measuring six nautical miles in length, was found to be up to 23 tonnes of gas condensate.
The Department of Energy and Climate Change says there is no evidence of any oil leak on the platform.