by Toby Helm and Heather Stewart

Alistair Darling is drawing up plans to face down the country's top bankers by taking the "nuclear option" of a windfall tax on their bumper bonuses as part of measures aimed at the super-rich.

The dramatic move, which was off the agenda just weeks ago, is under active discussion as the Treasury and No 10 try desperately to control the explosion of public anger over bankers' pay.

Government sources said Darling was keen to explore the option for introducing a windfall tax if practical problems – such as defining what constitutes a bonus – could be overcome.

It is also understood that Darling and his officials would only press ahead if safeguards could be built in to prevent widespread evasion. There is also concern in some parts of government about a potential backlash from businesses and the City.

The chancellor is expected to renew the onslaught on the highest earners in this week's pre-budget report (PBR) with a series of measures including moves to increase the number of people liable to pay inheritance tax and an increase in capital gains tax. Labour MPs and opposition parties are expecting the PBR to be the most political for years as Labour tries to contrast its own approach with Conservative party pledges to cut taxes for the most well-off.

Today, in a BBC interview, David Cameron will hit back at Gordon Brown's claims that his tax plans were "dreamed up on the playing fields of Eton", describing the attack at Wednesday's prime minister's questions as "petty" and "spiteful".

However, Darling is determined to balance any political message with clear signals to the financial markets that the government has a firm plan to halve the deficit over the next four years without jeopardising the fragile economic recovery.

Darling was persuaded to re-examine the case for hitting bankers with a one-off levy after a high-profile standoff last week with Royal Bank of Scotland, whose board threatened to quit if he vetoed seven-figure bonuses for its traders. But ministers now fear they will be vulnerable to charges of weakness if they fail to act as a string of other banks announce lavish payouts in the run-up to a general election, probably next May.

Sources insisted that no final decision had been made on a windfall tax for bonuses. But they hinted that plans being considered involved a one-off levy "targeted" at the biggest earners. This could include many of the 5,000 bankers who City minister Lord Myners cited as having earned at least £1m this financial year.

The government will also underline its determination to ensure financiers pay their share of the cost of the crisis by fleshing out its plans for an international "Tobin tax," to be levied on City transactions, raising tens of billions of pounds. Brown threw his weight behind the proposal last month, and No 10 has since been encouraged by growing global support, including from American Democratic majority leader Nancy Pelosi, who said last week that the proposal had "a great deal of merit".

Others measures likely to be included are those to increase, rather than cut, the number of people liable to pay inheritance tax, a move that will be seen as a deliberate ploy to discomfort Cameron who is committed to lift all but millionaires out of inheritance tax.

The chancellor will highlight the success of his recession-busting measures, claiming the stamp-duty holiday and help with mortgage interest payments has kept almost 400,000 people in their homes over the past 12 months.

Repossessions are running at half the rate of the early-1990s recession – thanks, Darling will say, to Labour's intervention.

With Britain still in the deepest recession for half a century, Darling will admit that the economy has contracted more sharply than he expected at the time of the budget – by 4.75%, instead of 3.5%. He will resist pressure from Bank of England governor Mervyn King to announce more "ambitious" plans to attack the Treasury's £175bn deficit, and instead seek to win credibility for his promise to halve it over four years.

The Treasury believes that "smarter government" – computerising claims for child benefit, shifting civil servants out of London, and scaling back government IT projects, for example – can help to save billions of pounds.

The Conservatives yesterday called for a moratorium on £100bn of existing and upcoming computer projects across central government. After Darling's speech on Wednesday, the Tories will try to deflect charges that they favour the rich by attacking Labour's latest tax-and-spend measures as political window-dressing that fail to address the need for a convincing plan to put the public finances in order.

In a prerecorded interview for BBC1's The Politics Show, Cameron said he was "not the slightest bit embarrassed" about his private schooling. "I never hide my background or where I'm from or anything about my life like that," he said. "My view is very simple... that what people are interested in is not where you come from but where you're going to."