A leading public finance expert has warned that the devolution settlement will be "seriously discredited" unless changes are made to the way the Scottish Executive's spending plans are approved.

Professor Arthur Midwinter, the former adviser to the Scottish Parliament's Finance Committee, said it would be unfair for the minority SNP administration to force its budget through parliament when it is outnumbered by opposition parties.

The executive is expected to present its budget bill to parliament later this year, when it will be scrutinised by the Finance Committee.

But while the committee can recommend changes to the budget, only a government minister can propose an amendment.

Writing in Public Finance magazine, Mr Midwinter says the system should be changed to better reflect the post-election parliamentary arithmetic.

He writes: "The executive's ability to reject parliamentary votes to alter the budget would be profoundly undemocratic.

"The will of parliament should prevail, and this proposal would strengthen parliamentary control of expenditure."

Mr Midwinter adds: "The election of a minority government provides opportunity for greater transparency in budget-making, by making the parliament the focus of decisions, not political deals in private meetings.

"For democratic accountability we need a budget process that reflects the parliamentary arithmetic. It is indefensible to continue with a mechanism that would allow a minority administration to ignore the wishes of the parliamentary majority, and seriously discredit the devolution settlement as a result."

Tavish Scott, the LibDem finance spokesman, yesterday welcomed Mr Midwinter's decision to enter the debate.

He said: "Arthur Midwinter is equally robust on all the parties' financial plans and therefore I believe that, as an impartial observer, his comments should be taken seriously."

Wendy Alexander, Labour's Shadow Finance Secretary, described Mr Midwinter's article as "an important intervention".

She said: "The SNP promised new politics. A proper test is whether their financial proposals can command the support of parliament."

The Conservatives have distanced themselves from the Labour and LibDem demands, but have also called for a review of the system.

Derek Brownlee, the party's finance spokesman, said: "What we need is much greater transparency in the budget process. That way, it can be put to proper scrutiny and the government held to account."

However, the signs last night were that the executive will resist any moves to change the rules which currently govern the budget process.

A spokesman said: "This government supports the view of the previous two administrations that the existing budget bill arrangements provide a robust method of responsible financial planning, ensuring that public services attract the financial support they require."