The report also shows that the poor are poorer here than in any other Western country for which figures have been collected and that a higher proportion of old people live in poverty than anywhere else in the West. Britain's child poverty rate, moreover, is second only to the United States.
This devastating indictment - which Claire Short, Secretary of State for international development, describes as "worrying" - contrasts sharply with progress in the fight against poverty in much of the rest of the world.
The Human Development Report, published by the United Nations Development Programme last week, concludes that poverty rates have fallen in over 100 countries over the past two decades and adds: "By the end of the 20th Century some three to four billion of the world's total population of 5.7 billion will have experienced substantial improvements in their standard of living."
The report has drawn up a new way of measuring wealth and poverty which compares not just incomes but purchasing power in all countries. By this measure the poorest fifth of Britons are able to buy less than their equivalents in any other major Western country. They have only two-thirds of the purchasing power of the poorest fifth of the people of the United States, Spain, Italy or Hong Kong - and are worse off even than those in Singapore, Israel and the Czech Republic.
Meanwhile the wealthiest fifth of Britons are among the best-off in Europe, outstripped only by the French, the Swiss and the Danes. They also enjoy about the same purchasing power as their peers in Japan, where the poor are almost twice as well off.
Britain's national wealth, the report shows, has been growing at almost exactly the same rate as in Sweden and Kenya. But the income of the poorest fifth grew 21 times as fast in the Scandinavian country, and four times as rapidly in the African one, as here.
The report adds that Britain is the only major Western country where poverty has "increased substantially" since the beginning of the 1980s. Up to then it had been falling, as in every other industrialised country. The change coincides exactly with Mrs Thatcher's coming to power: the proportion of poor people in "income poverty" jumped by nearly 60 per cent under her government.
Nearly a quarter of Britain's old people live in poverty, more than five times the rate in Italy, more than three times as in Ireland, and exactly the same proportion as in Taiwan.
And nearly a fifth of the United Kingdom's children are poor - about twice a many as in Taiwan or Italy, four times as many as in Belgium and six times as many as in Finland. Half of all Britain's single parent families live below the poverty line.
Last year's human development report showed that Britain is the most unequal country in the Western World. Dr Richard Jolly, Principal Co- ordinator of the report said last week: "Poverty and inequality have been rising in Britain relative to other countries in the industrialised world."