PLANS to replace air passenger duty in Scotland may have to be delayed, the Finance Secretary has told MSPs.

Air departure tax (ADT) was due to come into force in April 2018, with Scottish ministers pledging to cut the levy by 50 per cent by the end of this Parliament, ahead of abolishing the charge altogether “when resources allow”.

However Derek Mackay said its introduction could be pushed back because proposals to continue with an exemption for passengers who fly from Highlands and Islands airports require EU approval under state aid rules.

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He said discussions were ongoing with UK ministers to resolve the matter, but described their response so far as “disappointing”.

Airlines and airports expressed frustration at the potential delay.

Opposition parties accused Mackay of using the issue of EU approval as a means of kicking his policy to cut the tax “into the long grass”.

Scottish Greens co-convener Patrick Harvie said the Finance Secretary had come up with a “technical pretext to save his blushes”, with his party previously warning that they may not strike a deal on the SNP’s next budget if it included a cut to the tax.

Harvie said that the plan had “no social, economic or environmental credibility”.

Mackay had previously told MSPs the Scottish Government was of the view that, in order for the Highlands and Islands exemption to be compliant with EU law, it must be notified for approval to the EU Commission by the UK Government, as the EU member state.

However he said the Treasury had “serious concerns” about making such an approach, and would require Scottish ministers to “accept full liability” for all risks, including the historic operation of the tax, if the EU does not approve the exemption.

Mackay said: “The conditions the UK Government have sought to impose are clearly not acceptable.

“This government therefore finds ourselves placed in a deeply unsatisfactory position.

“We could choose not to introduce the exemption for Highlands and Islands flights. This would ensure ADT remains within EU state aid rules and avoid the need for notification, but would bring an unacceptable cost to the fragile economies of the Highlands and Islands.”

UK ministers have suggested the implementation of the tax could be deferred for “an unspecified time period”, Mackay said.

Meanwhile, the Scottish Government has proposed an amendment to its block grant from the Treasury to fund the exemption, which would allow the tax to be implemented on time.

“A delay is not my preferred option – I could only agree as a last resort, but I cannot see ADT put into operation with this significant uncertainty hanging over the Highlands and Islands,” Mackay said.

“I therefore urge the UK Government to step up to the plate, to recognise their responsibilities and to support our proposal which would enable ADT to go forward as planned without causing harm to the Highlands and Islands economies.”

A spokeswoman for the Treasury confirmed they would continue to help work towards a solution, but added that it was the Scottish Government’s responsibility to ensure the tax complied with state aid rules.

She said: “We are committed to the successful devolution of air passenger duty and UK Government officials will continue to work with their Scottish Government counterparts as they seek to develop a solution.

“It is disappointing that the Scottish Government has been unable to design a tax which is compliant with EU law.

“It is wrong for them to try and pass the blame for their unwillingness to take responsibility for their own tax.”

Scottish Conservative MSP Murdo Fraser said: “A cynic might conclude that today’s announcement has more to do with politics and the SNP’s desire to pally-up with the Greens again to get their budget through Parliament rather than with any legal technicalities.”

He questioned why the exemption, which has existed as part of air passenger duty without challenge for 16 years, was “suddenly now at this convenient point an insurmountable legal problem”.

The Finance Secretary said it remained the government’s policy to cut and then abolish the tax.