A health board is to divert ambulance patients to hospitals outside its area because of “critical” pressure on its largest hospital.

NHS Grampian said it had taken the “significant step” of declaring a critical board incident due to sustained and continuing demands on Aberdeen Royal Infirmary (ARI).

Patients requiring immediate life-saving care will still be dealt with at ARI, it said.

But others will be diverted to alternative hospitals when it is deemed “clinically appropriate” to do so.

Chief executive Adam Coldwells said some elective procedures and appointments would also be cancelled, but this would not include cancer treatment or diagnostic tests.

He said the board aimed to resume “business as usual” as soon as possible.

“Those already waiting at ARI will remain for assessment, and admission if required,” Mr Coldwells said.

“As always, those with the greatest clinical need will be prioritised.

“These options will assist us in the short term. We continue to work with our health and social care partnership colleagues to increase bed capacity to manage current demand across Grampian.”

NHS Grampian’s on-call medical director Hugh Bishop said the situation could last for several days.

“The volume of activity today at the moment is putting pressure on our system to such an extent that we’re not able to see our patients in as timely a fashion as we need to, to make sure they’re getting the right care in a timely way,” he told BBC Scotland News.

“For that reason we’ve taken a number of different steps and one of the most obvious ones, invisible to the public, is that some ambulances with appropriately selected patients are being diverted to other health boards for a limited period of time.”

The Scottish Ambulance Service said it was “working collaboratively with colleagues in NHS Grampian to manage the current pressures”.

A spokesperson added: “We would strongly encourage the public to phone NHS 24 on 111 for free, day or night, or to contact their general practice during the day before attending A&E for non-life-threatening health concerns.”

NHS Grampian has faced severe pressure in recent years, with staffing problems and multi-million-pound overspends.

There have also been serious issues with ambulances forced to queue up outside Aberdeen Royal Infirmary’s accident and emergency department.

Last month, the health board announced plans to reduce the number of ambulances waiting outside the hospital.

The scheme allows ambulance staff to take some patients directly into the initial assessment unit to avoid lengthy queues at the hospital entrance.

The Unison union has opposed the change, saying it will add to the pressure faced by hospital staff.