One of Britain's most prominent religious figures, Cardinal Keith O'Brien, has accused David Cameron of immoral behaviour and of favouring rich City financiers over those struggling on lower incomes.

O'Brien, Scotland's most senior Roman Catholic authority, said: "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.

"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."

O'Brien called for Cameron to introduce a Robin Hood or financial transaction tax on City dealings. "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," he told the BBC.

Last year Cameron and the chancellor, George Osborne, led Europe-wide efforts to stop France and Germany introducing just such a tax, arguing that it would be uniquely damaging to UK interests.

In a BBC1 Scotland interview, O'Brien said it was immoral "just to ignore" those suffering as a result of the credit crunch.

"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."

O'Brien said he was speaking in favour of a campaign by the Scottish Catholic International Aid Fund, which along with other religious and political groups has been campaigning for the introduction of a Robin Hood or Tobin tax that could raise £20bn each year from a 0.05% levy on financial transactions.

Figures from the Sunday Times rich list show that the assets of Britain's wealthiest 1,000 rose last year to £414bn.

A Downing Street spokesperson told the BBC the prime minister was "determined to help people" struggling with their finances. The government had "inherited enormous debt – but the prime minister is determined to help people who are struggling with the consequences of that", the spokesperson said.