Unions vowed yesterday to hit back at a “fundamentally unsafe” assault on oil workers’ shift patterns as BP announced it would cut investment by up to a fifth next year.

Wood Group chief executive Dave Stewart used Monday’s emergency oil summit in Aberdeen to reiterate bosses’ calls for workers to accept a change from current working patterns.

Workers who are currently given three weeks off for every two weeks worked on-platform have been told patterns will change to three weeks on, three weeks off.

This means employees, including those working night-shifts, would potentially have to work 21 consecutive days on the dicey rigs.

“There has been a decline in production efficiency,” Mr Stewart told delegates.

“The industry has been working too inefficiently for too long and those challenges have to be taken on board.”

But GMB regional officer John Kelly said: “In my view, this is fundamentally unsafe. Jobs in the offshore industry are some of the most dangerous.”

He warned that, including travel time and the potential for the weather to delay helicopter trips, workers could

ultimately spend more time on rigs than off.

Maritime union RMT, which represents platform workers previously covered by the Oil Industry Liaison Committee, has launched its “Three and Three, It’s Not for Me” campaign in response.

“Three and three has never been a recognised standard for production platforms,” RMT’s Jake Molloy told the Star.

“We’re not disputing that there are inefficiencies, but we’ve repeatedly provided alternative proposals for companies to work with.

“Nothing’s been looked at except making the workforce pay through the nose for management failings.”

And Unite union officer Tommy Campbell said: “All the evidence in any work environment shows clearly that the longer you are working on the job, the greater the risk of accident through fatigue.”

BP has contributed to over 1,000 job losses in the British oil industry and announced yesterday that it would slash investment by up to £4 billion.

Boss Bob Dudley said the company was adapting to “the new reality of lower prices.”