SNP ministers have been accused of presiding over a “complete shambles” after announcing a third delay to a manifesto pledge on boosting the air industry.

The latest hold up to Air Departure Tax - the proposed devolved version of Air Passenger Duty - raised fears the pledge will not be delivered within the current parliament.

It followed an admission it would not be ready for the 2020/21 financial year.

The news brought a furious reaction from the airline industry, which accused the SNP of not delivering a “cast-iron manifesto commitment” and told ministers “to be straight” with people.

The SNP promised to halve ADT by 2021 at a cost of £190m to lower costs to passengers and encourage investment in Scottish airports, then abolish it when funds allowed.

However the plan ran into trouble over a 2001 exemption from air tax granted to airports in the Highlands and Islands to help boost traffic in remote areas.

If the tax was devolved, to become ADT, there are fears this historic opt-out would fall foul of updated EU state aid rules, and costs would soar.

The standard tax rate is currently £26 for short haul and £172 for long haul flights.

Opposition parties claim the minority SNP administration is also dragging its feet to placate the Scottish Greens, who have helped pass the last three budgets, and who say giving the airline industry a tax break would add to carbon emissions.

The UK Government has suggested the Scottish Government cover the extra cost if the Highlands exemption ends, but Finance Secretary Derek Mackay has refused.

Mr Mackay set up an ADT Highlands and Islands Working Group last July in the hope a panel of outside experts could find a solution, however they too have failed.

Mr Mackay first announced a delay in October 2017, when he said ADT's “defective state” meant it would not be ready for the 2018/19 financial year.

In June 2018, Mr Mackay announced another year’s delay, saying ADT would not be devolved for 2019/20 because of problems over the Highlands exemption.

Four months later, Mr Mackay admitted he didn’t know when or how the “impasse” would end.

Mr Mackay’s deputy, public finance minister Kate Forbes, has now announced a third delay, admitting ADT will not be ready for the start of the 2020/21 financial year either.

She said: “The Scottish Government has been clear that it cannot take on ADT until a solution to these [state aid] issues has been found, because to do so would compromise the devolved powers and risk damage to the Highlands and Islands economy.

“A solution has not yet been found that would be ready for introduction at the beginning of the next financial year. This, taken together with the continued uncertainty around Brexit, means that that we have to defer the introduction ADT beyond April 2020.”

The delay was condemned in a joint statement by Tim Alderslade, Chief Executive of industry body Airlines UK, Derek Provan, CEO of AGS Airports Ltd, which owns Glasgow and Aberdeen airports, and Gordon Dewar, CEO of Edinburgh Airport.

They said: “The Scottish Government needs to be straight with industry. This was a cast-iron manifesto commitment and they have now failed to implement it two years in a row.

“In the meantime it is Scottish tourism and connectivity that is suffering, as we’ve seen with Norwegian pulling out of Edinburgh and lost routes at both Glasgow and Aberdeen.

“The message from airlines and airports is clear - either do what you have promised and get on with it sooner rather than later or be upfront with us that it is never going to happen.”

Tory MSP Jamie Greene said: “This is a complete shambles of a U-turn by the SNP.

“Despite an explicit manifesto commitment to reduce air departure tax at the last election, the Scottish Government has done everything possible to avoid honouring that promise. It has failed miserably to deliver this flagship policy after a dodgy budget deal with the Greens.

“The SNP continually calls for more and more powers to be devolved to them then asks the UK Government to either keep those powers or drags its heels to deliver them.

“Any suggestions that it supports Scottish aviation have lost all credibility.”

Other parties urged the SNP to tax cut altogether.

Labour MSP Colin Smyth said: “This is the latest delay in the SNP’s shambolic handling of Air Departure Tax.“Instead of delaying this tax cut which will help the wealthiest the most, they should be cancelling it for good.

“With school children striking around the world tackling climate change is at the forefront of our politics. Sadly, in Scotland, the SNP is planning to hand tax cuts to frequent flyers in a move which could damage the planet.”

Green MSP Patrick Harvie MSP said: “The SNP’s increasingly absurd attachment to the idea of a massive tax giveaway for the airlines is unfair to the majority of people who rarely fly, out of step with climate reality, and clearly undeliverable.

“They should stop pretending that it’s going to happen, and instead invest in the affordable public transport that people in every community in Scotland depend on.”

Scottish LibDem MSP Liam McArthur said: “While Ian Blackford joins other [Westminster] party leaders in meeting environmental activist Greta Thunberg, SNP ministers are sat squarely in the pocket of airline lobbyists.

“They are proposing pumping tens of thousands of tonnes of additional emissions into the atmosphere.

“Enormous tax cuts for the aviation industry will hurt our efforts to tackle climate change and leave less money available for public services.

“It’s time the Scottish Government abandoned this anti-environmental policy.”