The accelerating campaign for independence has received a timely boost after two of the UK airline ­industry's most senior figures expressed support for Alex Salmond's policy on the so-called air travel tax.

Willie Walsh, the chief of British Airways (BA), which employs around 1300 staff north of the Border, said independence would probably be a "positive development" for the flagship airline.

He is in favour of the SNP's pledge to halve and then scrap the controversial Air Passenger Duty (APD) in the event of a Yes vote in September's referendum. The surcharge is estimated by the Nationalists to cost the tourism industry around £200 million a year.

Mr Walsh's support for the Scottish Government's position on APD was echoed by Michael O'Leary, who runs the budget operator Ryanair. Mr O'Leary said: "There's no doubt most airlines would support the position of the Scottish Government in relation to the abolition of the APD, which does untold damage to Scottish tourism."

The support from two of the ­industry's most senior figures boosted the Yes camp, 24 hours after Edinburgh-based Standard Life revealed it had drawn up contingency plans to move some of its operation to England in the event of a vote for independence.

Asked if BA was also making contingency plans, Mr Walsh, who heads the airline's parent company, International Airlines Group, said: "No, because we'll continue to fly to Scotland. If anything, it might be marginally positive, as I suspect the Scottish Government will abolish APD, because they recognise the huge impact that tax has on their economy. So no, it's probably going to be a positive development, if it does happen, for British Airways."

Mr O'Leary said APD was "not a narrow or small issue" and had a "penal" effect on business.

He predicted the number of visitors to Scotland would double "in five or 10 years" if the duty were scrapped.

The First Minister has made it clear an independent Scotland would initially reduce APD by 50% to boost international air links and then abolish it "when public finances allow".

The tax is currently reserved to Westminster although calls have been made for Chancellor George Osborne to use his Budget later this month to scrap it.

Keith Brown, the Scottish Transport Minister, said: "Willie Walsh, the CEO of a group which includes British Airways among its portfolio, can clearly see the opportunities of independence. A boost to tourism and travel in Scotland will have a positive impact on growth."

A Yes Scotland spokesman said: "Mr Walsh is well aware of the damage Westminster is doing to Scottish tourism with crippling Air Passenger Duty and of the Scottish Government's pledge to cut and eventually get rid of it, so he speaks from a position of real knowledge. APD is yet another unwanted imposition on top of the hated bedroom tax and Trident that a Yes vote and the powers that it would deliver can free Scotland from."

However, a spokesman for Better Together said: "Breaking up the most successful economic, political and social union in history for the sake of a tax on holidays doesn't seem like the strongest argument.

"As the intervention from Standard Life made abundantly clear, leaving the UK would cost jobs here in Scotland."

Scottish Secretary Alistair Carmichael emphasised Scottish-based companies were now obliged to consider the possibility of independence and were "making plans to mitigate its dangers".

He noted the Financial Reporting Council, the independent regulator on corporate governance, had issued new guidance to companies on how they should address the issue of Scottish independence in their annual reports.