Unionist promises of more powers for Scotland if it votes No to independence have been called into question amid an ongoing split over the devolution of air tax.

Scottish Conservatives have proposed devolving air passenger duty (APD) but this has reportedly been ruled out by Tory leader David Cameron, Holyrood heard today.

Conservative MSP Alex Johnstone's proclamation that he is a "true devolutionist" was greeted by laughter from the SNP, who attacked the party's shifting stance on devolution.

Mr Cameron has said further devolution will require cross-party agreement but Labour's Mark Griffin said his party has yet to be convinced of the case for devolving APD.

Labour is concerned about the local economic effects of Scots taking more foreign holidays, he said.

Transport minister Keith Brown also attacked the Liberal Democrats, whose Scottish Secretary withheld APD from the Scotland Act 2012 despite the advice of the Calman Commission, which recommended its devolution with the backing of all the unionist parties.

Deputy First Minister Nicola Sturgeon has written to Chancellor George Osborne urging him to devolve APD in tomorrow's Queen's Speech.

Speaking at Holyrood, Mr Brown said: "I note the position taken previously by Ruth Davidson that APD should be abolished, and also the Liberal Democrats' position on federalism, so we may have the grounds for some consensus in the Parliament.

"But it's quite a confused picture because I understand yesterday, despite the fact that the Calman Commission which the Conservatives supported but hasn't taken any action on its own recommendation, Ruth Davidson felt it necessary to restate her support for the devolution of APD, although that has since been contradicted by a report in The Guardian today.

"She has also mentioned that she sought its abolition from David Cameron, who point blank refused, and what a perfect example of why we should have independence.

"In relation to the Liberal Democrats, we have had a statement from the Scottish Secretary that this would be happening, and lo and behold it's not happening."

The Scottish Government has proposed a 50% reduction in APD in the first independent Scottish Parliament, followed by its complete abolition once economic conditions allow.

Mr Griffin said: "We have thought long and hard about APD and we're still unconvinced about removing it.

"We have discussed through the Calman Commission and introduced it for debate in our Devolution Commission, and while we feel that APD is in need of reform, a 50% reduction then total removal just wouldn't be sensible without further consideration of the economic and environmental impact.

"We are not closing the door to further devolution to Scotland, but we think further decision-making is required before a decision like that is taken."

He added: "I agree there will be an increase in passengers coming to the UK, and that would be a benefit, but similarly there would be an increase in passengers leaving the UK, and so home-based revenue for tourism would surely be affected as well.

"How much would it cost the Scottish economy and the public purse if more Scots decide to go on foreign holidays rather than stay and visit UK destinations?"

Mr Johnstone said: "I am willing to hold out the olive branch and say that, yes, the Calman Commission said that APD should be devolved.

"The Strathclyde Commission, the Conservative Party's document that sets out what we are prepared to do in the event of a No vote, is a detailed document which goes into great length about the generous proposals for devolution that we will have in that event.

"Yet one small part of it, the part that says we would like to see APD devolved, is the one that the SNP is getting most excited about."

He added: "I, as a true devolutionist, who believes in decision-making right here in Scotland, am prepared to propose that we as two political parties with so much that separates us, reach out and link hands going forward together for the benefit of Scotland and its air passengers in order to secure in the long term a sound commitment that this evil tax will ultimately be a burden removed and one that will have to suffer no longer."

In her letter to Mr Osborne, Ms Sturgeon said: "The principle of transferring responsibility for APD has already been accepted by HM Treasury and put into practice to support the Belfast to Newark route.

"The Calman Commission recommended that responsibility should be devolved to the Scottish Parliament.

"Yesterday the Scottish Conservatives' Commission chaired by Lord Strathclyde states that APD should be devolved on the basis that Scotland would be better able to manage this tax locally.

"Lord Strathclyde's Commission also notes that there is no need for fresh legislation and the process is straightforward if both the Scottish and UK governments agree.

"Given this unequivocal recommendation and the broad support there is for transferring responsibility for APD to the Scottish Parliament, I very much hope you will act swiftly on the commission's recommendation and use the opportunity of the Queen's Speech to make a positive announcement that will be broadly welcomed in Scotland."