New rules are to be introduced on the size of oil workers travelling to and from offshore installations by helicopter.

Passengers with a shoulder width of 22 inches or more will be classed as «extra broad» and will have to sit next to a similarly large helicopter window.

It follows a Civil Aviation Authority (CAA) review of helicopter safety.

Concerns were raised about larger passengers being able to escape from a crashed aircraft quickly.

The changes come after a study revealed the average oil worker had risen in weight by close to 20% over the past 30 years.

There were concerns that size had become a risk factor in helicopter accidents, with larger workers finding it harder to leave helicopters during emergencies due to the size of the exits.

Now those classed as «extra broad» will have to sit next to a window with a diagonal width of at least 22 inches.

The offshore safety body Step Change said at least 25% of helicopter windows meet the new criteria.

It said there would be ongoing discussion about how, when and where passengers would be measured.

Les Linklater, team leader for Step Change in Safety, said: «The safety of the offshore workforce is our absolute priority and we believe this measurement criteria and strategy is the safest and simplest route to take.

«We have worked hand-in-hand with the CAA to measure the windows on every type of helicopter used in the North Sea and it is our understanding that at least 25% of windows are suitable for extra broad passengers.

«Our commitment to safety will guide our ongoing discussions as to exactly how, when and where offshore passengers will be measured.»

The move follows other changes to the helicopter safety regime — including strict new clothing rules for passengers introduced last week.