Nicola Sturgeon has warned David Cameron keeping the UK in the EU could hinge on a deal on Scottish devolution.
The first minister accused the UK Treasury of trying to “systematically reduce the Scottish budget” as talks over future funding arrangements for Holyrood raged on.
She has written to the prime minister to complain the UK and Scottish governments are not currently working on a “shared understanding” of one of the key principles behind proposals for transferring more powers over tax and welfare north.
She said: “As we approach an EU referendum, people anywhere in the UK have a right to know that what the UK Government says in a referendum campaign can be trusted and will be delivered.”
The two administrations have been locked in protracted negotiations over the fiscal framework for the Scotland Bill, which sets out how the block grant will be altered when MSPs get new tax-raising powers.
Yesterday it was revealed that Deputy First Minister John Swinney was preparing a fresh offer to the Treasury which concedes further ground.
Ms Sturgeon argued that any additional risk on being proposed by UK ministers through the fiscal framework would act as a “double whammy” of pain.
She said: “That is simply not acceptable. It would mean Scotland having to grow receipts from income tax much faster than the rest of the UK just to stand still.”
The SNP leader insisted the she supports the principle of taxpayer fairness, where Holyrood’s budget would not change because of UK spending on devolved areas.
Holyrood ministers had been working to a deadline of February 12 for a deal to be agreed Mr Swinney has asked Holyrood’s Devolution Committee to be flexibility in its timetable.
The PM’s official spokeswoman said: “We remain committed to implementing the Smith agreement in full. A deal has got to be fair to Scotland and it has got to be fair to taxpayers in the rest of the UK too.”