The Health and Safety Executive has ordered BG (formerly British Gas) to carry out urgent improvements to safety procedures.
An internal BG report, obtained by The Independent, highlights the seriousness of the first leak in the Rough gas field, which lies some 20 miles of Hull, last February and points to management and operational mistakes.
BG's report, by its safety and environment directorate, says: "There were no injuries but the high potential for major loss made it necessary to instigate a thorough independent investigation."
Unions claim the incidents, on 13 February and 20 May, bear an uncanny resemblance to the gas leak on the Piper Alpha production platform in the North Sea, almost exactly 10 years ago, which killed 167 oil workers.
Roger Spiller, head of MSF's North Sea section, said: "Our members are concerned that BG appears to have done nothing to sort out the problems."
The starting problem in both leaks appears to have been failure of a flange during maintenance work. Lord Cullen's inquiry into Piper Alpha found that that disaster was also caused by a flange failure.
BG confirmed it had hit trouble on Rough, which is used for the storage of gas to meet peak winter demands. A formal statement, released yesterday about the 13 February incident, says: "This occurred during maintenance and the leak was quickly controlled."
Of the 20 May incident, a statement said: "The platform immediately shut down. There were no injuries or damage."