When the middle-class daughter of former flight attendants who counts miners among her ancestors married the second in line to the throne last year, it was hailed as the ultimate breaking down of social class barriers.
Yet Kate Middleton's achievement in "marrying up" to become the Duchess of Cambridge is becoming increasingly rare, with more young British women choosing husbands from their own social class or lower, a new study has found.
Compared with their mothers, women in their 20s are less likely to marry men in a higher social class than their own, research by the IPPR think tank shows.
The authors suggest an entrenchment of social class and widening inequality over the past three decades have driven the phenomenon of "assortative mating", or picking a partner similar to yourself.
The proportion of women "marrying down" has also exceeded those "marrying up" for the first time.
Researchers analysed the backgrounds and marriage choices of women born in 1958, 1970 and 1981 to see how female aspirations have changed across the generations.
With post-war social mobility on the rise, women born in 1958 were more likely to "marry up" than "marry down".
About 38 per cent of women of this generation chose a partner in a higher social class, while 23 per cent married someone from a lower class. A total of 39 per cent married someone in the same class.
Madonna, who was born in 1958, may be a multimillionaire but has working-class roots and married film director Guy Ritchie, the son of an army officer who has noble ancestry.
For those born in 1970, the proportion of women marrying someone in a higher social class fell to 32 per cent, although this was still more than those "marrying down", at 23 per cent.
Those marrying someone from the same social class accounted for 45 per cent.
However, today's generation of brides, born between 1976 and 1981, is for the first time more likely to "marry down" than "marry up".
While 56 per cent marry in the same class, those choosing a spouse from a lower social class account for 28 per cent. Only 16 per cent of women are marrying men from higher social backgrounds.
One woman born in 1981 who, it could be argued, "married down" is Princess Anne's daughter Zara Phillips, who wed the middle-class rugby player Mike Tindall last year.
The IPPR suggests that one cause for the shift in marriage patterns is the changing jobs market since the war. In the 1950s and 1960s, de-industrialisation and the growth of women working in junior office jobs led to a trend of "marrying the boss", the report says.
But as inequality grew in the 1980s, with losses of blue-collar, middle-tier jobs, education became more closely linked to occupation, and social class began to "harden its grip on who people met and subsequently married".
Nick Pearce, IPPR director, said: "This new analysis shows how social class has tightened its grip on marriage in Britain.
"In the post-war period of rising social mobility, men and women were more likely to marry across class lines than they do today.
"This shift has implications for inequality, as high earners marry each other and then pass on the fruits of their combined success to their children."
The research also shows more women of the current generation are marrying partners three or more years older, with the largest increase shown in women marrying men seven or more years older, who account for a fifth of this generation of married women.
Mr Pearce said: "Age no longer seems to be a social taboo, with many more people marrying partners who are more than one or two years older than themselves than in the 1970s and 1980s.
"While governments have no business telling people who to marry, and have plenty of bigger economic inequalities to aim at, it is important for policymakers to understand these trends if they are to have a full understanding of what's driving the stagnation in social mobility."
The IPPR said the trends were important for understanding society today because, if more people marry within their own class, wider income inequalities are exacerbated.
The trend will also cause child poverty rates to increase. When better-off people marry each other, they are able to invest more time and resources on their children's development.
Recent OECD research showed that the earnings gap between the wives of rich and poor husbands in the UK has grown since the 1980s, from £3900 ($7557) in 1987 to £10,200 in 2004.
A report last year by the OECD found that income inequality among working-age individuals had risen faster in the UK than in any other industrialised nation since 1975.
The annual average income of the top 10 per cent of earners in 2008 was almost £55,000, nearly 12 times higher than those in the bottom 10 per cent, whose average income was £4700. In 1985 the ratio was eight to one.
But it is not just in the UK where marrying up is in decline: Across industrialised countries of the OECD, 11 per cent of the rise in inequality since the mid-1980s is attributed to "assortative mating".
In Sweden, an academic report last year concluded "marriage behaviour is polarising" and argued that "marital homogamy" (marrying someone of the same class) is resistant to policy efforts to increase social mobility.