The airport, which no longer routinely publishes its traffic figures, revealed it handled 9.7 million passengers in 2018 compared to 9.9 million the previous year.
It has been hit by the loss of several routes, including around ten of Ryanair’s, and US links such as to Philadelphia, while New York flights have been reduced to summer only.
The downturn in passengers, revealed today in a study into the economic impact of the airport, is in stark contrast to rival Edinburgh Airport, which grew by 6.5 per cent to 14.3million last year.
A Glasgow Airport spokesman said: “While the Ryanair announcement was in February last year, the full effect is being felt this year, but we expect to return to growth next year.”
The report, commissioned by the airport from economists York Aviation, said it contributed £1.44 billion to the Scottish economy.
It is forecast to rise to £2.57bn by 2040.
The study said passenger numbers were projected to increase to 12.8 million by 2028 and 16.8 million by 2040 – 400,000 higher than in the airport’s last master plan forecast in 2011.
An airport spokesman said: “The more recent figures take account of more up-to-date data and are therefore slightly different.”
The study indicated the airport supports more than 30,000 jobs across Scotland, with this number also potentially rising to 43,000 by 2040.
In 2017, the airport also handled more than £3.5bn in global imports and exports.
Managing director Mark Johnston welcomed the figures and used the opportunity to again call for progress on the proposed rail link to the airport.
He said: “This study confirms the huge economic benefits the airport generates for the city and Scotland each year.
“The findings are very encouraging and show that when Glasgow Airport succeeds Scotland shares the benefit.
“We have invested more than £130 million in our facilities since 2011 and have a strategy in place through our master plan to put us on the path to becoming a 17-million passenger airport by 2040.
“We can’t do this on our own, and it is imperative that through the ongoing support of our city and national partners we ensure Glasgow Airport continues to thrive and deliver for Scotland.
“To do this we also need to address the current constraints such as motorway congestion and delivering the planned rail link.”
Bob Grant, Renfrewshire Chamber of Commerce chief executive, said: “Delivery of the planned rail-link is an essential component of that strategy and will support Glasgow Airport as they continue to invest many millions for the benefit of our economy.”
As part of the investment into facilities, £8m is being invested to upgrade the airport ahead of Emirates operating the world’s largest commercial aircraft, the A380, on its Dubai route from April.
A new advanced manufacturing innovation district near the airport is also estimated to create up to 10,000 additional jobs.
Two tenants have already been confirmed in the £56m medical manufacturing innovation centre and the £65m national manufacturing institute for Scotland.
Finance Secretary Derek Mackay, also MSP for Renfrewshire North and West where the airport is based, said: “Renfrewshire benefits hugely from having Glasgow Airport on our doorstep, forming a vital part of our local community.”