Norman Baker has quit as a Home Office minister, complaining that working under Home Secretary Theresa May was like "walking through mud".
The Liberal Democrat MP accused the Tory Cabinet minister of viewing her coalition colleagues as "a cuckoo in the nest rather than part of government".
His decision comes days after he became embroiled in a fresh disagreement over drugs policy and accused the Tories of suppressing a report which backed his case for a review of the current law.
Lewes MP Norman Baker has responded to claims he is blocking new legislation to reduce carbon emissions from new cars, after Greenpeace activists unfurled a huge banner criticising him in the town today.
The Transport Minister said: "It is quite wrong of Greenpeace to make unfair and inaccurate allegations of this nature. I am not trying to water down this legislation.
"On the contrary – as always and as I have throughout my political career, I am trying to get the most sensible result for the environment. Greenpeace, in their campaigns, should challenge those who don’t care about the environment, rather than those who do.
"The Department for Transport is working hard to drive down emissions from vehicles, both through these complex negotiations in Europe, and elsewhere. We are rightly focussed on getting the best outcome from the 2020 targets – which is the main point of current negotiations.
"We are not in principle against setting a tough 2025 target, but to do so now risks setting industry an overly generous target. Greenpeace should withdraw these allegations."
Greenpeace activists have scaled a five storey building on Lewes High Street, unfurling a huge advertising banner demanding local MP Norman Baker takes action to limit carbon emissions.
The campaign group has criticised the Transport Minister for "blocking new legislation to reduce carbon emissions from new cars".
It says it has a leaked briefing in which Mr Baker calls colleagues to oppose a proposal by fellow Liberal Democrat Fiona Hall MEP to reduce CO2 emissions from all new cars to 70g CO2/km for 2025.
Greenpeace campaigner Sara Ayech said: “These documents reveal that there is a split at the heart of the Liberal Democrats over policy to reduce carbon emissions and save motorists money.
"Norman Baker should follow the progressive political lead of Fiona Hall and support laws that will help the environment, whilst putting an average of £400 a year back into the pockets of hard pressed motorists."
Now his day job maybe an MP- but in the evenings Norman Baker the Liberal Democrat for Lewes likes nothing more than jamming with his band! Today he releases his album called "Always Tomorrow" with his group The Reform Club.
One of the songs is called Brightling Church about the East Sussex Church of the same name which appears on the front cover.
Lewes MP Norman Baker explains why they're trying to cut down on the number of road signs across the country.
Can you spot the difference? Thousands of road signs across the country are being taken down in a bid to reduce what the Department of Transport calls "sign clutter". The department wants confusing road signs to become a thing of the past.
Lewes MP Norman Baker has pledged his support for state aid to be used in rolling out universal superfast broadband access.
The Liberal Democrat says councils should "move swiftly" to help fund the proposals, following the European Commission's decision to approve the Government's £530 million plan.
The coalition has committed to 90% broadband coverage by 2015, meaning better coverage for rural communities who suffer poor connections.
It will give Britain "the best coverage in Europe" and mean the creation of a "rural community broadband fund", but will cost £530 million.
More than £10.5 million will have to be provided be East Sussex County Council and Brighton and Hove, the councillor said.
Due to state aid being provided, the funding had to be approved by the European Commission.
Mr Baker said: “The decision by the European Commission to approve the coalition’s state aid for the roll-out of superfast broadband is excellent news. Constituents, particularly in rural areas, and local businesses are crying out for access to quicker broadband.
"I know from the letter I received from East Sussex County Council that getting approval from the EU was considered a final hurdle.
"That hurdle has now been jumped, and with the county council, hopefully, already down the road of procurement, there is no reason why roll-out should not now move forward swiftly.”