The headline rate of Capital Gains Tax will be cut, George Osborne said.
It currently stands at 28% but will now be cut to 20%.
The lower growth forecast for the economy means the Chancellor will have to raise taxes or make drastic cuts, the economic think-tank said.
The Budget seems to be a return to 'nudge, nudge' politics which looks at behaviour, the cost of dealing with it, and makes a calculation.
Around 20 Tory backbenchers wrote to the Chancellor, even before the Budget was delivered, to outline their concerns about the measure.