Nationalist ministers accused the Conservatives yesterday of wilfully using wrong figures to back up their claim that local income tax would add £300 a year to the tax bill of the average Scottish household.

The Tory figure contradicts the SNP administration's claim that taxpayers would be better off with a local income tax rather than the “unfair” council tax.

Widespread coverage of the Tory claim yesterday suggested that most Scottish newspapers are now firmly against local income tax, making it all the harder for Alex Salmond and his colleagues to win public support.

According to a study carried out by the Scottish Tories, two people on average earnings, living in a house with an average council tax, would be £289 a year worse off with a 3p-in-the-pound local income tax.

The Tory study said that the average couple would be worse off in all 32 local council areas in Scotland, with extra costs ranging from £62 a year in Moray to £758 in the Western Isles.

John Swinney, the Nationalist Finance Secretary who championed the tax, said yesterday that the Tories were using the wrong figures for average earnings in Scotland. He said: “The Tories assume two full-time earners in each household, despite this being entirely unrepresentative. In fact, the number of households in Scotland with two or more full-time earners is only 17 per cent.”

Mr Swinney added that, even assuming a couple were both working full-time, the most accurate average total earnings figure would be £44,938, rather than the £53,929 quoted by the Tories. Mr Swinney also attacked the Tories for assuming that those couples were paying council tax at less than the Band D average (£983 compared with £1,138) - an assumption that the minister said was “nonsensical”.

“Assuming a couple both on full-time earnings are living in an average Band D home (although the likelihood is that most would be above Band D), their council tax will be £1,138. On the basis of the 3p local income tax policy, they would pay £1,024, a saving of £114.”

The SNP said that the Tory figures amounted to “nothing more than an inaccurate apology for retaining the hated council tax. On average, compared with the council tax, a married couple without children will be £176.80 a year better off, a married couple with children will be £161.20 a year better off, a single non-pensioner will be £171.60 a year better off, a single pensioner will be £379.60 a year better off, and a pensioner couple will be £717.60 a year better off,” the SNP claimed.

However, notwithstanding its defence of local income tax yesterday, the SNP is finding itself increasingly on the back foot over the plan. The Tory attack came after serious doubts were expressed in Whitehall that the £400million Scotland receives in council tax benefit would continue to be paid if council tax were scrapped. The Nationalists say that it should be.

The Treasury has also said that local income tax is constitutionally illegal under the Scotland Act, and

Mr Swinney has not satisfactorily explained how he would fund the estimated £750million financial black hole that opponents say would be created because the new tax would not raise the same amount as council tax does.

The Tories said that they stood by their figures, which were from government statistics. A spokesman said: “Whether a household has two people on average full-time incomes or one person with above average income and one with below average income, the ‘Nat Tax' bill is the same.”

The Tories also challenged the SNP to say how much the average council tax would be if the £280million of efficiency savings needed to set a 3p rate were instead used to slash council tax.

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