Protecting the City while neglecting Britain's struggling families is "immoral", according to Scotland's senior Catholic.
Cardinal Keith O'Brien's attack on the coalition's approach to taxation policy is part of his bid to persuade ministers to accept "a small financial transactions tax".
He has written to David Cameron calling on the government to implement the so-called 'Robin Hood tax' which would place a 0.05% tax on bonds, stocks and derivatives sold in the City.
Cardinal O'Brien told BBC Scotland he believed the less well-off, including those who were being forced to give up their retirement homes or struggling to get on the property ladder, were being neglected.
"It is these people who have had to suffer because of the financial disasters of recent years and it is immoral," he said.
"It is not moral, just to ignore them and to say 'struggle along', while the rich can go sailing along in their own sweet way."
Mr Cameron vetoed the proposed EU treaty on fiscal integration last December partly because he could not accept proposals for an EU-wide financial transactions tax, which would disproportionately affect the City.
Traders believe the introduction of such a tax would push businesses away from the City, undermining London's status as a major global financial centre.
But campaigners want a Robin Hood tax to raise £20 billion a year. The money would be spent on poverty alleviation both in the UK and overseas.
"At this difficult economic time more needs to be done to help the poor, both at home and abroad, as they are the ones hit the hardest by the fall-out of the global financial crisis," Cardinal O'Brien, who is chair of the Scottish Catholic International Aid Fund, said.
"The banks and financial sector, which caused the problem, have a clear responsibility to pay their fair share.
"It is shameful that David Cameron is currently protecting his wealthy friends in the City by his opposition to this simple, fair and sustainable financial transactions tax."
His comments were dismissed by Downing Street, which pointed out in a statement that the last Budget had taken two million people on of the lowest incomes out of tax altogether.
Mr Cameron told BBC1's The Andrew Marr Show this morning: "I understand those difficulties, that is what fires me up, that is what gets me out of bed to work hard to do the right thing for my country and it's got to be about the long term."
David Hillman, spokesman for the Robin Hood tax campaign, said that the government needed to put the interests of all of Britain before those of the Square Mile, however.
He asked: "Why should poor people at home and abroad pay the price of the crisis while bankers who helped cause it rake in multi-million pound salaries?"