Oil and gas companies have been issued a warning by regulators over slow decommissioning of their sites in the North Sea.

Plans have been approved by the North Sea Transition Authority for the plugging and abandonment of wells over the next 50 years, which is a complex and expensive process.

Taking too long, or deferring work, adds to the cost and can mean that platforms continue to use power and release emissions even though they are no longer producing oil and gas.

Operators are legally required to decommission their platforms, pipelines and wells once they stop producing  and expect to spend about £24bn on decommissioning between 2023 and 2032.

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In its latest cost and performance update, the NSTA said that "the majority" completed much less work than originally planned.

Hundreds of wells will need to be decommissioned every year as more oil and gas fields shut down. However, oil and gas companies only achieved 70% of planned well decommissioning activities last year.

Although the level of inactive wells to be decommissioned is increasing, the availability of the equipment to do so is decreasing, meaning people and equipment could be lost to other regions if targets are not met.

Some companies are said to be deferring decommissioning in the hope that costs will fall in the coming years, but the NSTA said that meant losing people and equipment to regions offering secure, long-term contracts.

The body said it was "getting tough on operators who do not meet their regulatory obligations on well decommissioning".

Members of the NSTA’s directorate of regulation have commenced investigations relating to alleged failures to complete timely plugging and abandonment in line with approved plans.

Pauline Innes, the NSTA’s supply chain and decommissioning director, said: “With spending forecast to peak at £2.5bn per year in the current decade, decommissioning can ensure that the UK’s world-leading supply chain is equipped to help operators clean up their oil and gas infrastructure over the next 50 years and support the carbon storage sector, which will rely on many of the same resources.

“I am concerned that this huge opportunity to safeguard highly-skilled jobs and support the transition will be wasted if operators fail to tackle their well decommissioning backlogs.

"The supply chain wants to do this work, but it is not physically tied to the UK. Its skills and resources are in demand in other regions, and we are starting to see companies marketing their rigs elsewhere. Operators need to use the supply chain, now, or risk losing it.”