David Cameron will commit the Tories to a new tax on banks today as both main parties battle to convince voters they would claw back billions of pounds from the sector.
The Opposition leader will say the US-style measure is needed to ensure taxpayers are repaid in full for the bail-out of financial institutions.
His firm pledge comes amid renewed confidence of international backing for such a levy - although party sources indicated it would press ahead unilaterally with a more limited measure if not.
Chancellor Alistair Darling is expected to use next week's Budget to signal the Government's support for a global tax, although only as part of an international agreement.
The Financial Times reported that he would set out detailed options in Wednesday's statement but would insist money raised should go into the Treasury coffers and not be used for an insurance fund against future collapse.
In a speech this morning, Mr Cameron is expected to declare that the banks are one of the "vested interests" he is determined to confront.
"We had the biggest bank bail-out in the world. We can't just carry on as if nothing happened.
"In America, President Obama has said he will get taxpayers back every cent they put in. Why should it be any different here?
"So I can announce today that a Conservative government will introduce a new bank levy to pay back taxpayers for the support they gave and to protect them in the future.
"No, it won't be popular in every part of the City. But I believe it's fair and it's necessary."
Prime Minister Gordon Brown has been a leading advocate of some form of co-ordinated levy on the banks, which could bring in tens of billions of pounds a year from the financial services sector worldwide.
He was forced to abandon his preferred option - a "Tobin" tax on transactions - but hopes the IMF will throw its weight behind a global levy at its April meeting in Washington.
There are fears that taking unilateral action could lead to an exodus of institutions from the City.
Ministers have criticised Tory support for US President Barack Obama's proposals, which they say are specifically tailored to the American situation and would not work in the UK.
The FT said Labour's manifesto could also commit to diverting some of the proceeds into aid for poorer countries - in line with a mounting campaign for a "Robin Hood tax" on the banks.
Mr Cameron will use his speech to launch a fresh assault on Mr Brown over the BA cabin crew strike, renewing his accusation that the PM is unable to take on the trade union which is "bankrolling" Labour.
Mr Brown has condemned the strike as "deplorable" and called for it to be halted but the Tories have seized on the industrial action to highlight Labour's links with Unite.
The union has donated £11 million to the party in recent years and has links to many MPs and election candidates - sparking Tory warnings of a return to 1970s-style industrial strife.
Mr Cameron will say Mr Brown has "consistently given in to special interest groups" on several fronts and promise to "fight" vested interests if the Tories take power at the general election.
Vital public service reforms had been halted because the PM was "too terrified of upsetting the union barons, losing their votes and their money", he will say.
"Powerful lobby groups" secured the green light for a Heathrow runway despite environmental concerns and Mr Brown failed to stand up for taxpayers' interests after the bank bailout, he will say.
"Once again, under Gordon Brown the vested interests triumph and the people lose out.
"And now we see it again with the British Airways strike. This threatens the future of one of Britain's greatest companies along with thousands of jobs.
"But will the Prime Minister come out in support of those people who would cross the picket line? No - because the Unite union is bankrolling the Labour Party. So the vested interests triumph and the people - including those cabin crew staff who don't want to go on strike - suffer.
"From the BA strike to public sector reform and his approach to the City of London, Gordon Brown has consistently given in to special interest groups and shown that he is unable to deliver the change that the country needs."
He will hail former Tory prime minister Lady Thatcher's trade union reforms, privatisations and "right to buy" council house schemes as examples of how to take on "entrenched" interests.
A tieless Mr Cameron was introduced as "the next Prime Minister" by Justine Greening, Tory MP for Putney, Roehampton and Southfields, as he sat waiting to deliver his speech.
Appearing with his shirt sleeves rolled up, Mr Cameron was loudly applauded as he spoke in the affluent suburb.
Liberal Democrat economics spokesman Vince Cable said: "The other parties seem to be moving on to ground the Liberal Democrats have occupied for some time: banks must pay for the protection they enjoy from the taxpayer.
"The absolute key is that Britain must do this whether or not other countries act, because Britain is uniquely exposed to the risk of a bank collapse. The ratio between bank assets and GDP is far higher in Britain than in other Western countries.
"The Liberal Democrats have been very specific about how this crucial issue should be tackled, after extensive discussion with the City and others, and it is seriously worrying that both the Conservatives and the Government still do not seem to have worked out a specific proposal - long after the bank collapse and only a few weeks from an election."
City minister Lord Myners criticised the "ill thought-out" announcement and said any unilateral action would force bankers to quit the UK.
Lord Myners said: "This morning's ill thought-out Tory briefing has all the hallmarks of a plan made up on the hoof.
"This kind of tax on bankers needs to be international. As the Chancellor has made clear, the UK-only bank tax the Tories want is a sure-fire way of encouraging a banking exit from the UK - raising costs to borrowers, damaging Britain's competitiveness and costing tens of thousands of jobs across the country.
"The Tories keep getting the big judgments wrong. Dealing with the banks needs real answers.
"That's why we're at the forefront of international negotiations, tightening regulation and levering money out for small and medium-sized enterprises.
"We have a plan to get our money back and we will: by selling the banks we own at the right time and the right price."
The Conservatives denied putting UK competitiveness at risk - insisting that the scale of the levy would depend on the level of international support.
"We would not impose a massive tax unilaterally," a source said.
The party cited the support of Swedish finance minister Anders Borg - saying it was "crucial that taxpayers' interests be protected in the case of future financial crises".
Sweden introduced a "stability fee" in 2009 to pay for a fund which is expected to total 2.5% of GDP in 15 years' time.