THE Scottish Greens have threatened to vote down next year’s budget unless public sector workers get an inflation-busting pay rise - but admitted they don’t know the cost.

Co-convener Patrick Harvie said only the Scottish Government could calculate the sum because of the “ridiculously complicated” funding formula for Holyrood.

The Tories accused him of holding the minority SNP administration “to ransom” and dragging it to the Left.

Nicola Sturgeon has promised to lift the 1 per cent pay cap in 2018/19 but has yet to say by how much, while her finance secretary Derek Mackay has hinted at only a modest rise.

Mr Harvie’s demand is for a rise of more than 3 per cent based on current inflation.

The Fraser of Allander Institute has estimated it could cost £450m to give all public sector staff in Scotland a real terms rise, although £80m would be recouped in tax.

The Greens have also proposed hiking taxes on the better off to raise a further £330m.

Mr Harvie told the BBC’s Sunday Politics Scotland such a rise was a condition of Green support for the budget.

He said: “They [the SNP] have said they will lift the 1 per cent pay cap. That is not the same as a real terms increase.

"With inflation going up, we really need to push the SNP much further than they’ve gone on this. Lifting the pay cap would still imply a blow inflation increase. That means a real terms pay cut.”

Asked if the six Green MSPs would vote against the budget unless all public sector workers in Scotland got a pay rise of more than 3 per cent, the Glasgow MSP replied: “Yeah, and that’s the position that the trade unions are pushing as well.”

But asked how much it would cost, he said: “The Scottish Government is the only body that is able to produce a robust costing.

“We’re putting pressure on them to come out with those figures, let us know what it would cost, and that would feed through into the discussion we’re open to on tax.”

He said the Greens were ready to junk some of their previous tax proposals to strike a deal, saying parts of their 2016 manifesto “may need to be updated”.

Income tax in Scotland is currently levied at 20p above £11,500, at 40p above £43,000 and at 45p above £150,000.

The Green manifesto said it should be at 18p above £11,500, at 22p above £19,000, at 43p above £43,000 and at 60p above £150,000.

Tory MSP Dean Lockhart said: “Patrick Harvie’s list of unrealistic demands is getting ever longer. With the SNP desperate for their support many will wonder if the Scottish Government will ever stand up to Patrick Harvie’s ever fanciful requests.”

Labour MSP James Kelly added: “We welcome this commitment from Patrick Harvie to vote against a budget that doesn't deliver a real terms pay rise.

“In the last budget, the Greens supported cuts of £170m to local services in just this year - and it is encouraging to see Patrick Harvie is beginning to realise the cost of SNP austerity.”

At the Scottish Greens’ conference on Saturday, Mr Harvie said his party could not support a budget that included the SNP’s stalled plan for a cut to air passenger duty.

Delegates on Sunday backed the “progressive use of newly devolved tax powers” to pay for public services and said an SNP refusal to use Holyrood’s the powers was “unacceptable.”

The conference also passed motions calling for the devolution of immigration and asylum powers and an overhaul of the main Brexit legislation at Westminster.

At a separate conference in Edinburgh on creating a Greener economy, the former chair of the Financial Services Industry Lord Adair Turner backed the SNP’s plan for Scottish Investment bank to support innovative low-carbon companies.

He said: “All economies need to make an energy transition, to accelerate change towards low-carbon systems. It will need a joined-up public policy framework, and I am pleased to see that emerging in Scotland, and a big shift in patterns of investment.”

STUC General Secretary Grahame Smith said: “A Scottish Investment Bank should support the generation of good jobs and stimulate a new, green, industrial base for Scotland.”