BORIS JOHNSON’S costly proposal to raise the income tax higher-rate threshold from £50,000 to £80,000 would mainly benefit the wealthy, the Institute for Fiscal Studies (IFS) said today.

According to the think tank’s analysis, the Tory leadership frontrunner’s tax proposals, which would cost £9 billion, would mainly benefit the top 10 per cent of earners, as they would receive three quarters of the gains.

Interviewed today, Mr Johnson was more eager to put himself on the side of low earners, raising the threshold of wages that people earn before they pay national insurance contributions (NICs).

As usual, he avoided any details, but the IFS said that raising the threshold by £1,000 would take 600,000 workers out of NICs at a cost of about £3 billion a year. Currently earnings over £8,632 are subject to NICs.

IFS research economist Tom Waters said: “These are expensive pledges to cut tax.” He suggested that raising working and child-tax credits would be “much more effective and better targeted” than raising the NIC threshold.

Shadow chancellor John McDonnell said: “Johnson demonstrates it’s the same old Tory story of tax cuts for the wealthiest and continuing austerity for the rest.

“He’s proving that he wants to be a prime minister for the few, not the many.”

Mr Johnson also named former work and pensions secretary and former Tory leader Iain Duncan Smith — notorious for his punitive cuts to disability benefits, implementing the bedroom tax and conjuring up the reviled universal credit scheme — as his campaign manager.

Labour Party chairman Ian Lavery said: “Boris Johnson launched his campaign with a tax cut for the rich, then boasted about being the biggest defender of the bankers who crashed our economy.

“Now he has appointed the architect of Tory attacks on the poor, sick and disabled as his campaign chief.

“Johnson is for an elite few, not the many.”