SNP ministers have been urged to abandon a controversial tax break for the air industry after the levy involved was delayed for a second year.

In a written parliamentary answer, Finance Secretary Derek Mackay admitted Air Departure Tax - the devolved version of Air Passenger Duty - would not be ready in 2019-20.

Already postponed in 2018-19, the new tax has been held up by a dispute over maintaining a long-standing exemption from air passenger tax in the Highlands and Islands, which could fall foul of EU state aid rules.

The SNP has promised to halve ADT by 2021 at a cost of around £190m in order to lower costs to passengers and encourage investment in Scottish airports.

However last year Mr Mackay halted its devolution on the eve of the 2018-19 budget - coincidentally easing a deal with the Scottish Greens, who vehemently oppose the tax.

Mr Mackay said the Scottish and UK governments had continued to work on creating a new Highlands and Islands exemption.

But he said: “Despite these efforts, and combined with the continuing uncertainty as to how Brexit might affect aviation, it has become clear that aiming to introduce the tax at the beginning of the next fiscal year, April 2019, is not possible.

“The Scottish Government has a longstanding commitment to reduce ADT by 50 per cent, and we are doing all we can to work with airlines and airports to help grow the direct routes which are important for our tourism sector and Scottish businesses.

“While we work towards a resolution to the Highlands and Islands exemption, we call on the UK Government to reduce APD rates to support connectivity and economic growth in Scotland and across the UK.

Scottish Liberal Democrat leader Willie Rennie said the tax cuts should be abandoned.

He said: “Instead of using Brexit, the EU and the Highlands and Islands airports exemption as a pathetic excuse on ADT cuts, the Scottish Government should just abandon their anti-environmental policy.

“Cutting ADT will damage the environment and won’t help our battle with climate change. It will also hit our public finances at a time when we are facing a massive £1.7bn forecast downgrade. The SNP should finally give up and accept they are wrong on this.”

Green MSP Patrick Harvie added: “The SNP must be counting their lucky stars that they’ve found a way of getting out of their commitment to an ill thought out, evidence-free policy.

“They know that if they were to carry out their proposal to slash aviation taxes, something which is designed only to keep airline lobbyists happy, they would undermine their own climate change objectives as well as their ability to achieve a positive result of the next budget negotiations.

“Greens will continue to oppose this pointless corporate tax break in principle, and advocate for sustainable transport policies which put people’s regular public transport first, and the interest of airline shareholders at the back of the queue.”

Labour MSP James Kelly said: "The SNP government's handling of Air Passenger Duty has been a shambles from the outset - and as usual ministers are trying to point the finger at anyone except themselves.

"Attempts to give a tax break to frequent fliers while schools are lacking teachers and hospitals are lacking nurses tells you everything need to know about the SNP's priorities."

Loganair managing director Jonathan Hinkles said: “We are very disappointed to learn that plans for Air Departure Tax to be reduced have been yet further delayed.

“We continue to lobby for the current exemption for flights departing from the Highlands and Islands to be reciprocated so that it applies on flights to the region from larger airports like Glasgow and Edinburgh.

"Although we understand the position in which the Scottish Government has unfortunately found itself, it looks as though this will now hold back developments that would grow both tourism and business travel within Scotland, and that delay will be detrimental."

A Flybe spokesperson added: “We firmly believe that plans to reduce the level of APD, which as a UK-levied tax is one of the highest of its kind in the world, are essential for Scotland’s future success in remaining an attractive destination for business and tourist travellers.”

A HM Treasury spokesperson said: "We have agreed with the Scottish government to devolve Air Passenger Duty, and we are helping Scotland design its own tax. It is up to the Scottish government when this happens."