Liberal Democrat leader Nick Clegg has described this year's Budget as a "green cop-out".
Speaking in the Commons on Wednesday, Clegg said Alistair Darling had not done enough on green taxes.
"This Budget was widely trailed by the Treasury as the greenest ever," he told MPs.
"But at the first sign of political difficulty the government has run away by postponing the petrol duty increase until October."
The Lib Dem leader said Darling had performed an "act of political ventriloquism".
"The chancellor is the prime minister's creature, struggling to clear up a mess left by his boss under instruction from Number 10," argued Clegg.
"This is not a Budget for the environment. It's a Budget driven by fiscal incompetence and political desperation," he added.
Clegg also criticised the government's announcements on capital gains tax, saying these were "mired in chaos".
"It is a meagre, tinkering Budget which gives precious little help to the poor but maintains special treatment to the rich," he said.
"A Budget designed to fill a black hole masquerading as good for the environment. A Budget which will not make Britain fairer. A Budget which is a green cop-out."
On Darling's child poverty pledges, Clegg dismissed the "meagre, piecemeal reforms" to the "chaotic tax credit system", which he said would do nothing to help the government meet its target of halving child poverty by 2010.
He described the measures on fuel poverty as "too little, too late", saying that limiting regulations to pre-paid meters "doesn't go nearly far enough".
"This is not a green Budget, this is not a people's Budget. This is a tinkering con-trick Budget that protects the rich and abandons the poor," he added.
