Workers at two airports have begun the first of two 12-hour strikes in a dispute over pay and pensions.
Members of the Unite union at Glasgow and Aberdeen Airports walked out at 4am on Friday.
It follows a breakdown in talks about pensions and pay with owner AGS Airports.
Unite said Glasgow Airport had withdrawn from negotiations over its proposal to close its pension scheme to existing members, which the union says broke an existing Acas agreement made in 2016.
A further 12-hour strike is planned at both airports on June 10 and a four-hour walkout at Glasgow on June 14.
AGS Airports said “robust contingency plans” will be in place to ensure that both airports are fully operational.
However there were delays at security at Glasgow Airport with queues “longer than normal”.
The airport tweeted: “We apologise to our customers for the delay at security this morning.
“Following the commencement of industrial action and transition to our contingency in security, we have experienced longer than normal queues.”
Those involved in the strike action include security staff, fire and airfield support staff.
It is thought around 400 people are on strike at Glasgow and some 300 at Aberdeen.
Unite regional industrial officer Pat McIlvogue said: “Unless the company get back round the negotiating table, we cannot rule out further dates being added to those already announced in a dispute solely manufactured by AGS management.”
An AGS spokesman said: “We have been in talks since January and, despite attending Acas, there continues to be no willingness whatsoever on the part of Unite to engage in any constructive dialogue.
“We made a significant improvement on our initial pay offer, which was increased from 1.8% to 3% in line with demands.
“This was rejected by Unite without any further consultation with members, and the union continues to deny them the right to make a decision on the offer of 3%.
“In regards to our final salary pension scheme, it is simply unaffordable with the cost to the company due to rise to 24.7% per employee.
“We tabled a generous compensation package for the remaining members, which again was rejected by Unite without first consulting its members.
“As always, we remain open to continuing dialogue with Unite to resolve this dispute.”