A report claiming a reduction in Air Passenger Duty (APD) could boost the Scottish economy by £1bn was based on research by a firm that has worked for major aviation companies.

Edinburgh Airport's dossier was put together by York Aviation, which counts anti-APD airline Ryanair amongst its clients.

Green MSP Alison Johnstone said she was not surprised that an "aviation-friendly consultancy" was involved in a "PR exercise" for big business.

APD is an excise duty levied on passengers when they fly from UK airports, with its cost dependant on the class of travel and the destination.

It generated around £234 million from flights in Scotland in 2012/13 - around 8% per cent of the UK total.

Supporters of the tax argue that APD helps cuts carbon dioxide emissions, but critics say it is a burden for families.

Airlines dislike the charge as it makes flying more expensive.

Edinburgh Airport, which handles around 10 million passengers a year, last week published a report calling for APD to be cut by 50%

It said halving the tax would create 4,000 jobs and boost the economy by £1bn within five years.

Without this action, the report warned that Scotland could lose out on around 1million passengers and cost the economy up to £68million in lost tourism every year.

APD is due to be devolved to the Scottish Parliament and the SNP is committed the cutting the tax.

However, the "independent" consultancy that compiled the report has links to the multi-billion airline industry.

York Aviation, a "specialist firm of air transport consultants", has worked for Easyjet, Cityjet and the European Airline Association.

It also provides "ongoing advice" to Ryanair, whose chief executive Michael O'Leary is a staunch supporter of scrapping APD.

He said last year: "APD should be repealed up here in Scotland. It is a major disincentive to airlines, and it is one of the reasons why we struggle to develop the business at Prestwick. It turns off a lot of visitors to Scotland.

"Services that have been lost in recent years would come back at three, four, five times if APD was scrapped."

Johnstone said:

"Highly profitable airlines don't pay a penny of tax on fuel and they are failing to pay for the pollution they create. It doesn't surprise me that an aviation-friendly consultancy is involved in a PR exercise trying to making life easier for these big businesses.

"Let's focus instead on boosting domestic tourism, and designing a tax system that shifts responsibility onto highly profitable firms so they pay for their pollution. Let's also focus on what really matters to the tourism sector in Scotland - issues around skills, broadband and reliable public transport within Scotland."

Dr Richard Dixon, the Director of Friends of the Earth Scotland, said:

"The air travel industry has form for using its own pet consultants to produce wild figures for how wonderful a decrease in APD would be or how terrible an increase would be. A few years ago they told us it would be a disaster when APD was increased but the number of people flying continued to grow even through the recession. Their latest claims are just as unbelievable. "

A spokesman for Edinburgh Airport said: "We chose York Aviation to undertake this report as they are one of the leading research firms in this area, and because they work with other airlines and airports. The economists there understand the aviation market intimately and how Air Passenger Duty works to frustrate its growth, making flights to and from Scotland more expensive and reducing the number of direct connections available to Scots travellers."

A spokeswoman for York Aviation said: "We do relatively little work for airlines."