Exclusion zones have been put in place around the Elgin platform in the North Sea, which has been suffering a serious gas leak since Sunday.
Coastguards said shipping was being ordered to keep at least two miles away and there was a three-mile exclusion zone for aircraft.
A cloud of gas was reported to be surrounding the platform, which is located 150 miles (240km) off Aberdeen.
Workers from a second platform and drilling rig have been removed.
Shell has moved 120 non-essential staff from the Shearwater platform and Hans Deul drilling rig, about four miles from the Elgin, because of the drifting gas.
The oil giant said the move was a "precautionary measure".
Total, which operates the Elgin platform, said the situation was stable but it had not yet been able to identify the source of the leak.
A sheen of between two and 23 tonnes of gas condensate, and measuring six nautical miles in length, has been reported on the water nearby, and Total has activated its Oil Pollution Emergency Plan.
Jake Molloy, of the RMT union which represents offshore workers, said many had been watching the Old Firm game between Rangers and Celtic when the alarm was raised on Sunday afternoon.
He said: "Just as it finished a PA announcement went out to say everyone was to go to the muster point and this was not a drill.
"The stand-by vessel had identified the sea boiling, as it were, below the installation - suggesting there was gas coming up there and there was some kind of vapour cloud sitting on the surface of the sea."
David Hainsworth, health, safety and environment manager for Total E&P UK, told BBC Scotland's Good Morning Scotland programme the situation was "stable" and there had been no major change overnight.
He said: "We've been monitoring overnight. There seems to be no evolution in the rate of gas release.
"The area was over-flown twice yesterday by spotter planes to look at the sheen on the sea - that is about the same, at six nautical miles in length."
Mr Hainsworth said all the power was turned off when the platform was evacuated to minimise the risk of igniting an explosion.
He added: "Clearly there is a risk of ignition and a fire.
"We've taken away all the usual sources of ignition such as electrical power but yes there is a possibility.
"We believe it is low but you never say never."
Mr Hainsworth said they were evaluating the options for dealing with the leak.
Options include drilling a relief well - a process which could take many weeks.
Another possibility could be to carry out a "dynamic kill" - pumping heavy mud into the well to suppress the flow of gas.
Mr Hainsworth said there was a possibility that the leak could stop of its own accord.
He said: "I would say the best case scenario is that the gas is not very productive from this area and that it dies off over the coming days and weeks."
Dr Simon Boxall, an oceanographer at Southampton University, told BBC Scotland that this was not a deepwater drilling rig and platform but it was unusual in that they were drilling down 5km (3.1 miles) into the sea bed.
He said: "It is a very deep well. The gas they are bringing up is what we call sour gas.
"That gas has a high proportion of hydrogen sulfide and carbon dioxide and that makes it very flammable and quite poisonous.
"So the big problem they have got is dealing with a very combustible gas - unlike Deepwater Horizon where we were dealing with crude oil which ironically is very difficult to light sometimes."
Dr Boxall said it would be tricky to get close to the leak.
"I am guessing they are going to tackle it from beneath the surface to start with," he said.
"On the one hand the Total spokesman said there was no gas bubbling through the sea and yet the observer talked about the sea boiling.
"It seems unusual that it is not bubbling through the sea and that is going to add further complications of hydrogen sulfide going into the water - certainly causing widespread poisoning in the vicinity of the rig."
Dr Boxall said it was a very focused and localised issue but one which would be very difficult to tackle.
All 238 workers were removed from the Elgin installation and the nearby Rowan Viking drilling rig by helicopter on Sunday.
Shell confirmed on Monday evening it was flying non-essential personnel back to Aberdeen from its Shearwater platform and the nearby Hans Deul drilling rig.
The UK Department of Energy and Climate Change (DECC) has been monitoring the situation and said there was no indication of a risk of significant pollution to the environment.