Iain Duncan Smith has said he would sacrifice his Cabinet career for the sake of keeping Britain out of the European Union, as the bitter battle within the Conservative Party continued to rage.
Prime Minister David Cameron led an assault on pro-Brexit campaigners, accusing them of wanting to take "the gamble of the century" with the UK's future on the basis of only "extremely vague" proposals.
But the Work and Pensions Secretary - one of five Eurosceptic Cabinet ministers to have broken ranks to oppose the Government's position - said he was more committed to securing Britain's exit from the 28-nation bloc than completing his welfare reforms.
"If my face no longer fits, my face no longer fits. My big passion is welfare reform. But Europe goes over everything," he told The Sunday Telegraph amid suggestions of a post-referendum reshuffle purge of some leading rebels.
Mr Duncan Smith - a veteran of the rebellion against then premier John Major over the Maastricht Treaty - complained that ministers were undermining party unity by "briefing off" about the fate of colleagues.
And he pledged to defy a ban on "leave" campaigning ministers being shown official papers and briefings related to the campaign as Whitehall's top civil servant was summoned by a parliamentary committee to explain the move.
"I must have the right to continue to look at this. Constitutionally, I am in charge of that department," he said.
Cabinet Secretary Sir Jeremy Heywood faces a grilling by the Political and Constitutional Affairs Select Committee on Tuesday over his edict that it was "not appropriate or permissible" for officials to provide briefings, write material for speeches or supply access to official papers in relation to the referendum campaign to ministers opposing the official line.
Sniping within the party continued unabated into the weekend, with Mr Cameron and Chancellor George Osborne making barely-disguised attacks on London mayor Boris Johnson, one of the highest-profile pro-Brexit Conservatives.
Mr Osborne - hailing a warning from fellow G20 finance ministers that a UK divorce from Brussels could "shock" the world economy - said the issue was "deadly serious" for people's jobs and finances and not "some amusing adventure into the unknown".
The Chancellor in turn faced a backlash from Tory predecessor in Number 11 Nigel Lawson, who dismissed the G20 warning as "absurd".
It came as Philip Hammond was reported to have called arch-Eurosceptic backbencher Sir Bill Cash a "total s**t" for publishing confidential legal guidance about Mr Cameron's EU renegotiation deal.
A Foreign Office source said there should be a parliamentary investigation into why the EU scrutiny committee chaired by the veteran MP disclosed the advice prepared for European Council president Donald Tusk despite Mr Hammond handing it over in confidence.
The Mail on Sunday said he angrily confronted Sir Bill in Westminster during a break in a committee hearing.
An FCO source noted that the legal advice confirmed the deal was "legally binding and irreversible" but said Commons authorities "may now look into" what appeared to be a breach of rules allowing the Government to provide advice in confidence to committees.
The PM is on a whistlestop tour of the UK, warning voters against what he calls the "leap in the dark" of voting to leave the EU on June 23.
Writing in the Sunday Telegraph, he said: "When the people campaigning for "out" are asked to set out a vision outside the European Union, they become extremely vague. It's simply not good enough to assert everything will be all right when jobs and our country's future are at stake.
He challenged them to set out what the trading relationship would look like, how long the economy would face uncertainty while it was negotiated, how joint-security arrangements would be replaced and how Britain's role and influence in the world would be maintained.
"With so many gaps in the 'out' case, the decision is clearly one between the great unknown and a greater Britain. A vote to leave is the gamble of the century. And it would be our children's futures on the table if we were to roll the dice," he concluded.
At least 130 of the 330 Conservative MPs have publicly declared their intent to defy Mr Cameron and back "leave" in the referendum campaign.
Europe Minister David Lidington said the UK could be plunged into 10 years of damaging uncertainty.
"You would be in complete limbo and I think what that would do for the pound and for business confidence would be very serious indeed. It could last a decade," he told The Observer.
Mr Johnson faced accusations of "flip-flopping" after he ruled out the possibility of a vote to leave on June 23 leading to a better offer from Brussels and a second referendum - a position dismissed as "total fantasy" by Mr Cameron when it was floated by former Tory leader Michael Howard.
Mr Johnson previous assertion that "EU history shows that they only really listen to a population when it says 'no'" was widely seen as backing a fresh vote.
But he told The Times: "Out is out. What I want is to get out and then negotiate a series of trade arrangements around the world."