The UK North Sea's dwindling oil and gas reserves are pushing firms to tap unstable reservoirs at high pressure and extreme heat, while safety checks and maintenance are behind schedule, a North Sea rig auditor who works for the industry said.

French oil major Total is battling to stem a gas leak at its North Sea Elgin platform after a series of technical failures that industry sources say reflect wider lapses across Britain's offshore industry.

The auditor, an engineer and a union official said a range of measures designed to prevent a leak must have failed on Elgin, allowing gas to escape to the surface.

"There is a worrying backlog of maintenance on safety-critical equipment, including release valves, pipelines and sub-sea fail-safe devices," said the auditor, an oil industry professional with more than a decade's experience of safety systems and procedures. My experience in this region is that if you scratch beneath the surface, things get quite scary quite quickly," he said.

He said some North Sea rigs designed in the 1960s and 1970s were "falling to pieces" after exceeding their production lifespans, while more modern platforms were lagging well behind scheduled maintenance programmes.

Another source at a major oil company said safety still ranked high, but low gas prices - at about half their levels before the 2008 financial crisis - forced operators to weigh 'loss of life risks against loss of production risks.'

With rising operating costs and lower revenues, companies have put pressure on facilities to produce more fuel in order to break even, which means reducing the number of safety checks that could interrupt production.

The UK's offshore regulator, the Health and Safety Executive, has previously identified maintenance backlogs in successive asset integrity reviews, noting that maintenance on safety-critical equipment was especially poor.