THE SNP’s manifesto pledge to give a tax cut to air passengers is in doubt, after the Finance Secretary admitted he didn’t know when or how it would be delivered.

Derek Mackay said plans to devolve and slash air passenger duty were at an “impasse” because of a problem with EU state aid rules that could continue even after Brexit.

Asked at Holyrood’s finance committee when the tax would begin, Mr Mackay said: “I can’t answer that because the solution has not been found yet.”

The SNP said at the 2016 Holyrood election it planned to halve the devolved version of air passenger duty at a cost of £190m a year by 2021, and abolish it when funds allowed.

However the scheme quickly ran into trouble over the exemption enjoyed by airports in the Highlands and Islands to help them boost traffic.

If the duty was devolved, to become Air Departure Tax (ADT), there was concern this historic opt-out would fall foul of EU state aid rules, and costs would soar.

The standard tax rate is currently £26 for short haul and £156 for long haul flights.

Opposition parties have suggested the SNP is dragging its feet on the issue, as the Scottish Greens would boycott any budget that contained ADT, putting the government in jeopardy.

But at Holyrood’s finance and constitution committee Mr Mackay took the unusual step of revealing Scottish Government legal advice on ADT to insist that was not the case.

He said his government’s assessment was that the Highlands & Islands assessment was likely to breach EU state aid rules, and the UK government did too.

That compliance issue meant the Scottish Government’s law officers would not give approval to legislation containing the exemption going before Holyrood.

Mr Mackay said he asked the UK government last year, as the relevant EU member state, to notify the European Commission to ask for a Highlands and Islands exemption for ADT.

However the Treasury expressed “serious concern” about doing so, as they did not think it was EU compliant, and would only refer the matter to Brussels if the Scottish Government accepted all financial liabilities, including years of “historic non-compliance”.

Mr Mackay told MSPs: “Would you seriously expect me to sign up to taking on the historic liability for potential non-compliance of an EU Commission matter? Of course you wouldn’t.”

He said that if airlines in Scotland had to repay support it would have a “horrendous impact on the Highlands and Islands”, and so the UK and Scottish governments had then tried to trash out a solution between them, but without success.

Both governments say the other is responsible for finding a solution.

Asked if he was avoiding ADT to avoid trouble with the Greens at budget time, he insisted his government had been “working very hard” to find an answer, but in vain.

He said: “We cannot implement laws in Scotland that are contrary to EU law. Now, we don’t know how the world changes post-Brexit, but as it stands right now we cannot continue the Highlands and Islands exemption like-for-like.

“To impose the tax for the first time on the Highlands and Islands will have a… catastrophic impact on the Highlands and Islands. That’s why this issue is so important to have resolved.”

He said: “We are at an impasse with the UK Government… We are trying very hard to resolve this issue but I’m not going to sacrifice the Highlands and Islands of Scotland by imposing the tax upon them when we’re genuinely trying to work on a solution to this issue.”

He said a Highlands and Islands working group was now examining all possible options.

Asked when the tax was likely to be devolved, Mr Mackay said: “I can’t answer that because the solution has not been found yet.”