Energy Secretary Chris Huhne has ramped up coalition tensions over the future of the 50p top rate of income tax with a warning to the Conservatives against "helping their friends in the City".
The Liberal Democrat, a former City worker before entering Parliament, signalled his party would block any attempt to axe the levy on Britain's highest earners if it went before MPs.
But his comments come as Work and Pensions Secretary Iain Duncan Smith insisted it would be scrapped.
"On the 50p tax rate George Osborne and the Prime Minister both made it clear it was never forever so it is just a matter of when they decide that actually it has done whatever it has got to do in terms of getting the deficit down, that was always the position of the Government," he told BBC 1's Andrew Marr Show.
In an interview with Prospect magazine Mr Huhne said: "If the cut in the top rate of tax is just a way of helping the Conservatives' friends in the City to put their feet up, then forget it."
He added: "They are simply not going to get the votes in the House of Commons."
Mr Huhne called for the Conservatives to focus on lifting the lowest paid out of tax, branding it "nuts" that workers on the minimum wage were taxed at all.
"The top priority for tax cuts, if there is spare money, has to be lifting the hard-working low paid out of income tax altogether," he added.
"In the long run, it is nuts to tax someone on the minimum wage, and we should aim to lift thresholds above it.
"I have no ideological attachment to a particular tax rate. But if we are to cut the top rate of tax from 50 pence there has to be a cast-iron economic justification. During tough times, we have to be all in this together."