Ministers have launched a consultation on plans to cut air passenger duty (APD) in Scotland.

APD is being devolved to Holyrood, where the administration has committed to halving the rate from 2018.

Under the SNP's plans, the 50% reduction would begin in April that year and be delivered in full by the end of the next parliament in 2021.

The tax would be abolished completely when resources allow, ministers have said.

On Monday, the Scottish Government launched a consultation on its plans to reform the tax, which they describe as one of the most expensive of its kind in the world.

They are seeking views on how the reduction should be structured and how the tax should be operated to help boost Scotland's economic competitiveness.

The consultation will run for 12 weeks until June 3.

Launching the plan at Edinburgh Airport, Deputy First Minister John Swinney said APD "continues to act as a barrier" to Scotland's ability to secure new direct international routes - and maintain existing ones.

He said: "Scotland is already an attractive destination for business and inbound tourism, and it is important that we continue to open Scotland up to key and emerging markets in order to further capitalise on the opportunities that exist.

"We want to be consultative and collaborative, as we have been with the new fiscal levers already devolved to Scotland.

"This policy consultation allow us to take the next step and begin the process of developing a new approach that helps deliver our objective of sustainable economic growth."

Edinburgh Airport's chief executive Gordon Dewar described APD as a "regressive tax that hampers growth".

Amanda McMillan, chief executive of AGS Airports Ltd, which owns both Aberdeen and Glasgow airports, said: "APD is the highest form of aviation tax in the world and its reduction, and eventual abolition, will undoubtedly play a major role in strengthening Scotland's connectivity."

Views are also being sought on the environmental issues which need to be considered, ministers said.

Mike Robinson, of Stop Climate Chaos Scotland (SCCS), said it makes no sense to scrap APD.

He said: "This is one of the highest polluting yet least taxed global industries. This cut will further incentivise people to use the least sustainable transport option.

"Air Passenger Duty generates £230 million per year for the Scottish Government so, with constrained public finances, it's hard to see how this can be justified.

"At the same time, Scottish Government has cut spending on measures to tackle climate change in its recent budget. So the priorities are somewhat confused to say the least."

Scottish Green co-convener Patrick Harvie said: "In light of the latest climate change wake-up call, this is a pretty brazen announcement by the SNP.

"Persistence by the Scottish Greens resulted in ministers admitting that their APD policy would increase climate change emissions. After four years of failed targets, this policy simply does not stack up."

Willie Walsh, chief executive of International Airlines Group (IAG), voiced concern about the possible impact of APD changes on airports in the North of England.

He said: "The Scots are right to start getting rid of Air Passenger Duty. But reducing APD in Scotland will undermine airports in northern England as passengers make a dash for the border to pay less tax.

"This will seriously threaten the Chancellor's Northern Powerhouse and lead to a domino effect across the UK."

The easyJet airline said halving APD in two years' time could see it increase its flights to and from Scotland by almost a third, carrying more than a million additional passengers.

UK director Sophie Dekkers said: "EasyJet has long campaigned for the removal of air passenger duty. We know that its impact is most keenly felt in Scotland where passengers flying to and from other parts of the UK pay £13 on each flight."

She added: "If there is a 50% cut in 2018 we would expect to increase our flights to and from Scotland by around 30%.

"EasyJet is proud to be Scotland's largest airline and this would mean the current 5.5 million passengers we carry each year could increase to over seven million.

"This would deliver more services and routes for passengers in Scotland, including to European cities without a current direct connection, as well as the economic benefits of a larger easyJet operation in Scotland."