Around 500 patients could miss their NHS appointments as many flights from the Western Isles are suspended for six weeks.
Loganair has cancelled some services in the region due to industrial action at airports.
NHS Western Isles said it was informed by email of the suspension at 4.15pm on Friday, March 3, and the news had come at the “worst possible time”.
The health board said staff were working to try and minimise the impact on patients who rely on the flights for appointments however it stressed there were “limited options”.
“Whilst there would never be a ‘good’ time for a service reduction, this action has come at the worst possible time”Gordon Jamieson, NHS Western Isles chief executive
Services from Inverness to Stornoway and Benbecula are affected for the six-week period beginning on March 17.
Unions representing employees at airports operated by Highlands & Islands Airports Ltd (HIAL) are currently engaged in strikes and work-to-rule action.
Loganair bosses said the work-to-rule action is difficult to predict as staff cannot be covered through overtime or shift swaps.
It is estimated that around 230 patients who require travel to Inverness for medical appointments could be affected.
Arrangements for those well enough to travel by ferry are being made with them being urged to book through their local patient travel office.
Those unable to attend via ferry will have their appointments rescheduled.
It is also believed an additional 270 patients who were scheduled to attend an appointment on-island with a visiting consultant could be impacted.
This includes appointments including for ophthalmology, orthotics, ENT and urology.
NHS Western Isles said on Wednesday that there was no capacity in either Glasgow or Edinburgh to accept additional patients from the islands.
However, there will be limited inter-island flights between Benbecula and Stornoway to ensure that patients travelling from Uist to Stornoway on Tuesday for chemotherapy will be unaffected.
Blood and chemotherapy deliveries will also be unaffected.
NHS Western Isles chief executive, Gordon Jamieson, said: “With the exception of the pandemic, this action potentially presents us with the most significant disruption to patient services that we have experienced.
“Whilst there would never be a ‘good’ time for a service reduction, this action has come at the worst possible time when there is already significant pressure on the NHS, which makes it difficult to secure alternative options for patients.
“Our staff across the service are working with colleagues at NHS Highland to ensure that, where there is an alternative arrangement, that we rapidly put that in place.
“Whilst there will be a workaround for some patients, we are conscious that some patients will miss important appointments, and we continue to explore all options to avoid, as far as possible, any adverse impact. Patients whose appointments will be affected will be contacted by our staff directly, to discuss individual options.
“We would offer our apologies to patients for the unprecedented disruption to services that the suspension of flight services will cause for a prolonged period and would reassure everyone that we will continue to work with patients, partners on the mainland and with Scottish Government colleagues to minimise the impact, where we can, for individuals and families.”
A spokesperson for Loganair said: “We completely understand that this unprecedented step will be unwelcome news to communities who depend on the air services Loganair provides. It’s a step that we are taking with the utmost reluctance and only after careful consideration of all other options.
“We have sadly concluded that it’s simply not realistic to continue our efforts to provide services between HIAL airports when the action short of a strike is intended to disrupt and counter those efforts at every turn.
“The suspension provides advance – even if unwelcome – certainty around which our customers can adjust travel plans, as opposed to facing the risk of on-the-day flight cancellations or significant delays.
“We hope that the period of suspension – initially through to April 30 – will provide time and space for the parties to this dispute to meet and reach a resolution, enabling these long-running services to resume thereafter.”