Another senior Tory has joined the party's growing backbench revolt over proposed rises in Capital Gains Tax (CGT), warning it risked "punishing the virtuous" and "destroying aspiration".
Former shadow home secretary David Davis argued such a move may not even raise more money, citing evidence from the US which shows a rate cut actually boosts public coffers.
His comments come amid fears the new coalition Government could raise the current 18% levy on certain windfalls such as share and second home sales to the same level as income tax - up to 40% and 50%.
Writing in the Daily Mail, Mr Davis said for the really rich, CGT was "essentially a voluntary tax".
Hardest hit would be people reaching retirement who are selling off assets that they have saved years for - something they have little option over, he added.
The Haltemprice and Howden MP wrote: "Unless it is very carefully designed, the plan to increase CGT will not only fail to raise the money needed, it will cost money.
"It will penalise hard work and saving. Far from taxing the rich, it will simply tax the elderly at their point of maximum vulnerability - when they enter retirement."
On Wednesday former Tory minister John Redwood also spoke out against increases, saying they would punish entrepreneurs and savers.
In an open letter to the Treasury, he said: "(CGT rises) would leave long-term savers, people owning buy to let properties, and people with savings for retirement which are not held within a pension fund having to pay substantial tax."
The Prime Minister's official spokesman said: "The position on CGT is that there will be an announcement in the Budget. There are some words in the coalition agreement, but you will have to wait until the Budget."