A think tank has accused the Scottish Government of presenting "selective" data in its Budget documents and has called for a review of the process.

The Fraser of Allander Institute claimed the Government often uses data apparently designed to support its arguments, and has set out a six-point plan for reform.

The institute said possible reforms include a return to multi-year budgeting, a "radical reworking" of the structure and approach of the Budget document, and more information on long-term commitments, constraints and liabilities.

Further suggestions include changing the time of the Budget so it follows the new UK Autumn Statement and includes increased time for Scottish parliamentary scrutiny, emphasising outcomes instead of reporting changes in portfolio spend, and a more transparent presentation of the new fiscal framework.

In a blog post, the institute said: "The information in the Budget isn't particularly easy to understand. And the selective data that the Government presents often appears designed to support their arguments rather than to help inform debate.

"An improved budget process is a necessity. With the Scottish Budget now dependent not simply on a block grant from Westminster but on devolved taxes, debates are only going to get even more complex. We urgently require a process that is more fit for purpose."

It continued: "The draft Budget document is confusing and overly complex. It is difficult for even seasoned public finance professionals like ourselves to follow.

"The Budget document needs to be clearer on the split between factual information/data and policy/political interpretation. At the moment it is a muddle between a policy document and the authoritative source of budget data."

Scottish Conservative shadow finance secretary Murdo Fraser said: "(Finance Secretary) Derek Mackay should be embarrassed at these damning criticisms of the way in which he tries to spin his Budget.

"As the institute states, it is time for a budget process that is fit for purpose, not one manipulated for the SNP's political advantage."

A Scottish Government spokesman said: "The Scottish Government budget presents a detailed picture of our spending plans that support our economy, invest in our public services and use our new tax and welfare powers wisely.

"It is important that the budget accurately sets out what resources are available to the Scottish Government and the areas where we have discretion over spending decisions.

"The presentation of the budget regularly evolves in response to the changing powers of the parliament, new legislation and feedback from the finance committee.

"We have already established the Budget Review Group with the Parliament, which includes external stakeholders, to ensure the budget process is fit for our new tax powers and the changing timetables of the UK budget whilst continuing to provide opportunities for the appropriate level of parliamentary oversight and scrutiny."