Britain's most senior Roman Catholic, Cardinal Keith O'Brien, has accused the prime minister of acting immorally by favouring the rich ahead of ordinary citizens affected by the recession.
The cardinal also denounced David Cameron's opposition to a "Robin Hood tax" on financial institutions.
And he urged Mr Cameron not just to help "your very rich colleagues".
Mr Cameron said that he understood how difficult things were for families and that more jobs were needed.
The prime minister told the BBC's Andrew Marr Show: "I understand how tough it is for people in our country right now, if you're trying to make the household budget work out, if you've got petrol and diesel at the prices that they are now.
"I understand how difficult it is when you've got job losses in the public sector and you need the private sector to expand faster - we need more jobs.
"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."
The cardinal also said it was immoral "just to ignore" those who were suffering as a result of recent financial disasters.
In a BBC Scotland interview, he said: "My message to David Cameron, as the head of our government, is to seriously think again about this Robin Hood tax, the tax to help the poor by taking a little bit from the rich.
"The poor have suffered tremendously from the financial disasters of recent years and nothing, really, has been done by the very rich people to help them.
"And I am saying to the prime minister, look, don't just protect your very rich colleagues in the financial industry, consider the moral obligation to help the poor of our country."
The UK government has opposed the unilateral introduction of the "Robin Hood" tax, which would impose a small tax levy on large transactions of currencies, bonds and shares. It argues jobs and investment would be lost overseas.
But the cardinal said he believes that position is immoral because, he maintains, it overlooks the needs of the poorest in society and those of the less well-off.
He said: "When I say poor, I don't mean (only) the abject poverty we see sometimes in our streets.
"I mean people who would have considered themselves reasonably well-off.
"People who have saved for their pensions and now realise their pension funds are no more.
"People who are considering giving up their retirement homes that they have been saving for, poverty affecting young couples and so on and so on.
"It is these people who have had to suffer because of the financial disasters of recent years and it is immoral.
"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."
The cardinal was speaking in support of a campaign by the Scottish Catholic International Aid Fund (SCIAF) which says the billions of pounds raised by levying a financial transaction tax in the UK could be spent helping the poor and vulnerable at home and abroad.
The aid agency estimates a tax of 0.05% on major financial transactions, such as the trading of stocks, bonds and derivatives, would raise £20bn each year in the UK alone.
It wants that money to be spent tackling poverty at home and internationally, as well as on helping those in developing countries, whose lives are being affected by climate change.
SCIAF's campaigns officer, Lexi Barnett, said: "Cardinal O'Brien is joining a growing movement of high-profile figures calling for a Robin Hood tax to be put in place both in the UK and internationally.
"David Cameron and his government should do what is right.
"They should change their policy and implement the Robin Hood tax immediately so that the banks and financial institutions start paying their fair share to help those hit hardest by this crisis."
The Downing Street spokesman also said that the government had "inherited enormous debt - but the prime minister is determined to help people who are struggling with the consequences of that".
He added: "That's why the last Budget took 2m people on the lowest incomes out of debt altogether and from April 2012 pensioners will see the largest ever cash rise in the basic state pension."
The interview with Cardinal O'Brien will be broadcast on Sunday Politics Scotland at 12:30 BST on BBC1 Scotland.