Scotland faces massive cuts in public services unless the executive comes up with nearly half a billion pounds over the next three years to fund a planned freeze in council tax bills, The Herald has learned.

Senior council sources estimate that ministers will need to find an extra £70m every year in order to allow local authorities to provide the current level of services while not increasing council tax charges.

The SNP administration wants local authorities to freeze the council tax at current levels for the next three years, by which time it hopes to be able to replace it with a local income tax.

According to estimates seen by The Herald, to achieve this councils will need an additional £70m in the next financial year to cope with inflationary factors such as salary increases and fuel bills.

In the next year, the extra funding required will be £140m, rising to £210m in the third year. This means that overall, the additional funding required from the executive will be £420m.

Last night, opposition parties called on the executive to "come clean" and explain how it planned to find the extra money.

Wendy Alexander, Labour's Shadow Secretary for Finance and Sustainable Growth, said: "The SNP's chickens are coming home to roost. They must come clean and put a cost against their promise to freeze council tax.

"Given that they have little chance of getting their plans for a local income tax through the parliament, local government needs to know where it stands.

"The SNP will know their budget for the next three years in the autumn. Local government deserves the same certainty and that means a promise to fill the £420m funding gap."

Scrapping the council tax and replacing it with a local income tax of 3p in the pound was one of the central planks of the SNP election manifesto.

John Swinney, Cabinet Secretary for Finance and Sustainable Growth, is touring councils at the moment to set out his plans for local government.

But senior local government figures contacted by The Herald have expressed their concerns at the amount of additional funding they will need to meet the executive's plans to freeze council tax at current levels.

Pat Watters, president of the local authority umbrella body Cosla, said: "In this spending review we are equal partners with the executive. We are in discussion across all aspects from the money we get in, the amount of ring fencing, level of regulation and outcome agreements.

"The debate on freezing council tax cannot be reduced to the single issue of a specific cash figure to meet the freeze. However, the figures The Herald has obtained seem realistic and would roughly chime with our own calculations on this."

Tavish Scott, the finance spokesman for the Liberal Democrats, said yesterday: "The government will have to explain how they are going to balance the books. Many of us have concerns about the sums adding up and these figures illustrate that there is a black hole and parliament will want to know how that is going to be filled."

A spokesman for the executive said: "The Finance Secretary has had extremely constructive discussions with local authorities about working to secure a strategic agreement on the delivery of public services through local government, reducing the vast amount of regulation under which councils operate, strengthening the role of local authorities in local governance, and freezing the council tax.

"He is visiting the majority of local authorities to discuss this new, more positive relationship, and discussions about freezing the council tax are taking place in that positive context."