The North Sea oil industry needs to take a "reality check" and understand that current shifts which amount to staff working 20 weeks a year are no longer sustainable, according to one of the North Sea's biggest employers.

Unions have complained that changes in shift patterns from the current two weeks offshore followed by three weeks of leave to three on, three off will lead to longer hours and compromise safety.

But Dave Stewart, managing director of Wood Group UK which employs 12,000 workers including 5,000 offshore, said the industry has been working "too inefficiently for too long".

"Half of the employees work two on, three off offshore rotation which is effectively 20 weeks a year," he said at an oil and gas summit in Aberdeen.

"There has been a decline in production efficiency. Some see two on, three off as quite inefficient because of five-shift rota patterns.

"My own take on it is that the industry has been working too inefficiently for too long and those challenges have to be taken on board."

John Taylor, regional industrial officer for Unite the union, said recent shift changes and redundancies were announced without consultation, and called for greater engagement from the industry on future changes.

"You cannot have a situation where you make people redundant, force people to work longer hours, cut back on maintenance - we already have a maintenance problem - and say that you are working safely onshore," he said.

"That is totally, utterly unachievable."

Mr Stewart said: "I fully agree with John's point that we have to engage the workforce in that discussion.

"But I think we have to get a reality check and I don't think it's unreasonable to be asked to work more than 20 weeks a year."

Malcolm Webb, chief executive of industry body Oil & Gas UK, said: "While there is never any room for complacency on safety, I feel confident that there will not be any compromise on safety during this downturn.

"In any event, it makes absolutely no business sense for that to happen."

Mr Webb added: "Of course we need to take the workforce with us.

"In the end it all comes down to people, so it's hugely important that we do that.

"I don't know about individual cases but what I do know is that this is an industry that values its workforce, is approaching this whole difficult situation of a reduction of numbers - and there are going to have to be some people who are going to lose their jobs as a result of this - it's being done, as far as I can see, in a proper and sensitive way and engaging the workforce.

"I fully agree that that is the way that it must be done."