Nicola Sturgeon is under pressure to top up a £500 "thank you" bonus for NHS and social care workers after her challenge to Boris Johnson not to tax the money backfired spectacularly. One of Scotland's most eminent economic think tanks said the best way for workers to receive the entire sum would be for the Scottish Government to pay a higher "gross" amount, which would reduce to £500 on payment of income tax. The Fraser of Allander Institute (FAI), based at Strathclyde University, said there would be no additional cost to the Scottish Government as it controls income tax on earnings north of the Border, so the additional money would be paid back to them. In her SNP conference speech on Monday, Ms Sturgeon challenged Mr Johnson to create a special tax exemption for the payment, saying the Covid "heroes" deserve to pocket "every penny". After the Treasury pointed out SNP ministers had the power to "gross up the payment", Ian Blackford, the SNP's Westminster leader, accused the Prime Minister of trying to "punish" NHS and care workers. But the FAI said the SNP's exemption demand "opens the possibility of endless future lobbying for tax-exempt bonuses" and "would gift higher rate taxpayers a significantly larger tax break than basic rate taxpayers." In a damning intervention, they said there was a "healthy dose of politics" in Ms Sturgeon's challenge to the Prime Minister and what the Scottish Government "really wanted" was an example of the devolved tax powers "apparently not working."