The Liberal Democrat minister Vince Cable faced acute embarrassment last night after privately suggesting he could "bring down" the Coalition Government by quitting the Cabinet.
His unguarded comments shone a spotlight on the tensions behind the power-sharing deal with the Tories.
The Business Secretary, regarded as one of the Liberal Democrats least comfortable with the compromises made by the Coalition, revealed his frustrations in a conversation with undercover newspaper reporters posing as constituents.
He was recorded telling them that the Liberal Democrats had been pushing for a "very tough approach" to curbing bankers' bonuses, which had been opposed by "our Conservative friends".
He went on: "I have a nuclear option. It's like fighting a war. They know I have nuclear weapons, but I don't have any conventional weapons.
"If they push me too far then I can walk out and bring the Government down and they know that. So it is a question of how you use that intelligently without getting involved in a war which destroys all of us. That is quite a different position to be in and I am picking my fights – some of which you might have seen."
He reportedly criticised the speed at which the Coalition was trying to rush through changes in the health service and local government, warning it was in danger of getting out of control.
Mr Cable remarked: "We are trying to do too many things actually. Some of them are Lib Dem inspired, but a lot of it is Tory inspired. The problem is not that they are Tory inspired, but that they haven't thought them through. We should be putting a brake on it."
He said the cutting of child benefit for higher earners had been done in a "rather cack-handed way". He also hinted that winter fuel payments to the elderly could still be cut – despite assurances to the contrary by the Prime Minister, David Cameron.
"They haven't done the winter fuel payments, but that's coming, I think," he told the reporters.
Last night Mr Cable said: "Naturally I'm embarrassed by these comments and I regret them.
"I have no intention of leaving the Government. I am proud of what it is achieving and will continue to play my full part in delivering the priorities I and my party believe in, which are enshrined in the Coalition agreement."
The gaffe will leave Mr Cable, who was preparing for a bit of light relief over Christmas with an appearance on Strictly Come Dancing, vulnerable to Labour mockery. It also bolsters his position as the keeper of the Liberal Democrat conscience within government.