"A plastic bag charge is welcome news, but let's not get carried away," spokesman Craig Bennett said.
"If Nick Clegg wants to champion the environment he must do much better - starting by allowing Lib Dem peers to back a 2030 target for decarbonising the power sector when the Energy Bill reaches the House of Lords."
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Theresa May has launched her bid to become the next leader of the Conservative Party.
David Cameron's successor is likely to make the decision on whether to expand Heathrow or another London airport.
Michael Gove is to stand for the Conservative Party leadership, saying he does not believe Boris Johnson can "provide the leadership".