THE Scotland Bill has hit its biggest obstacle yet after Holyrood and Westminster clashed over a demand for a future veto once crucial details become public.

When the Scottish Parliament is handed new income tax powers the block grant will be reduced accordingly, but the detailed formula for this is still a long way off.

Finance Secretary John Swinney told the final evidence session of the Scotland Bill Committee that Holyrood must have a final right of approval once these details are known, and without that they would be compelled to vote down legislative consent for the Scotland Bill.

But later at the same panel considering the Westminster legislation at Holyrood the Secretary of State for Scotland, Michael Moore, rejected the demand for an amendment providing for this final approval, known as a Joint Commencement Order, to be inserted into the Bill.

Mr Moore, flanked by Scotland Office Minister David Mundell, was pressed on the claim from Mr Swinney and Parliamentary Business Secretary Bruce Crawford that the lack of such a provision could force Holyrood to reject the whole Bill.

“I am not persuaded the Joint Commencement is the right way to go,” insisted Mr Moore, although he promised to consider carefully the committee’s final report.

Mr Swinney said of such orders: “We need a proper and necessary ability to protect the Scottish public interest. As of today I could not tell the Scottish public that the Bill might not be to their detriment.”

The Finance Secretary had opened by saying the Bill represented a missed opportunity and posed real risks to Scotland’s finances. He said: “There is no debate that the actual mechanism for adjusting the block grant has yet to be decided. We are therefore in the dark about how to forecast the effects, never mind the inherent uncertainties in forecasting.”

He said “this feels like being asked to sign a blank cheque” and proposed that Holyrood should have a vote on the “commencement procedure” rather than just Treasury Ministers.

He added: “The block grant adjustment is not the only remaining issue to be settled. The sharing of the costs of implementation, accountability of Her Majesty’s Revenue and Customs, and detailed arrangements for the Scottish Rate of Income Tax, are also matters on which the governments need to reach agreement. All those negotiations would benefit from the commencement arrangements we propose, and we hope that this committee will support.”

Mr Crawford called the Scotland Bill “a complete shadow of what Westminster and this Parliament could have come up with,” citing the proposals on borrowing powers, Corporation Tax and power over the Crown Estates.

He said that in May voters had “chosen ambition over business as normal”. The Westminster Ministers’ repeated riposte was to point to the result of the previous May when the pro-Calman parties won their mandate.

Mr Crawford said: “We cannot accept an approach that says take a blank cheque and keep our fingers crossed. Our view is that both governments should now get down to serious negotiation. We are fast running out of time allowing both governments to say to their respective parliaments that these proposals are acceptable.”

There were feisty exchanges between Conservative MSP David McLetchie and Mr Swinney, and later between the SNP’s Stewart Maxwell and Mr Moore as proceedings adopted increasingly partisan lines.

Mr Moore claimed that as the “high-level principles” of how the various mechanisms would work had been established there was no need for commencement orders, prompting Mr McLetchie to say that this proved talks of blank cheques was wrong.

Mr Mundell, the country’s only Tory MSP, said: “I do not believe the Scottish election result earlier this year was a mandate to strengthen this Bill.”

The end of the lengthy session was dominated by attempts by the convener, the SNP’s Linda Fabiani, to extract an absolute pledge from Mr Moore that there would be no attempt to insert a clause in the Bill at Westminster dictating the terms of the planned referendum.