A gas blast on a North Sea oil platform was caused by the "catastrophic" failure of pipework that had not been properly inspected in more than 30 years.

The Boxing Day incident left the Brae Alpha badly damaged and came less than seven months after a gas leak caused by a corroded piece of equipment which also had not been properly inspected.

Ageing pipework failed "catastrophically" and caused an uncontrolled high-pressure blast of gas which caused "significant damage to the surrounding area", according to an investigation by the Health and Safety Executive (HSE).

Workers said the area hit by the blast was "like a bomb-site", according to RMT union organiser Jake Molloy.

HSE experts told Brae Alpha operator Marathon: "You had failed to remove the insulation of said pipework for inspection purposes since its commissioning in 1983 despite your own corrosion under insulation strategy issued in 2014, which recommends the removal of said insulation at least every 12 years for inspection purposes."

They added: "These failures exposed personnel on the Brae Alpha installation to an unacceptable risk of serious personal injury from fire and explosion."

Nobody was injured in the incident but Marathon has been ordered to make improvements for breaching offshore fire prevention regulations.

A spokesman for the US firm said: "Marathon Oil is cooperating fully with the HSE,and is taking immediate steps to comply with the improvement notice.

"Many of the prescribed remedial actions are already well under way. The safety and welfare of our workforce continues to be our first priority.

"Production remains shut-in on the Brae Alpha installation while we continue our investigation and will remain so until we are confident it is safe to resume operations. At this time, we are unable to estimate a timeline for this."