Shell has been warned by the UK North Sea safety watchdog after a section of well casing weighing more than three tonnes plummeted 15 metres on the Brent Charlie platform in the northern North Sea.

Shell has been served with an Improvement Notice by the Health & Safety Executive (HSE) after the incident on 23 August.

Nobody was hurt but inspectors said it had "significant potential to cause a major accident”.

Work was taking place to decommission wells using a hydraulic workover unit (HWU) when lifting equipment failed immediately after a section of casing strings had been cut.

HWUs are tower-like structures used instead of a conventional derrick to carry out well maintenance, especially when deck-space is limited.

The casing fell about 15 metres from the top of HWU work basket, a gantry area where workers manning the equipment are located.

HSE said Shell’s assessments for cutting and recovering the casing failed to identify the risk of lifting equipment failure and the consequences of an object being dropped from the workbasket.

“As such you did not determine likelihood and severity of harm occurring, who might be harmed, and any suitable control measures necessary to manage the risk from a dropped section of casing,” HSE said.

Shell has been given until 29 October to comply.

A Shell spokesperson said: "Although no one was injured, the incident was concerning. Across our business the health and safety of our people remains our highest priority.

"We are currently working to address the Improvement Notice and to learn any important lessons from what happened.”