Prof Coleman said migration has become the “primary driver of demographic change”

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It lays bare the effect of decades of immigration and claims that white Britons will be in a minority by 2066.

A mass influx of migrants has given the UK the fastest-rising percentage of ethnic minority and foreign-born populations.

The report – which also reveals the huge impact of Labour’s open-door policy to immigration between 1997 and 2010 – says foreigners and non-white Britons living here will double by 2040 and make up one third of the UK population.

Report author Professor David Coleman said: “On current trends European populations will become more ethnically diverse, with the possibility that today’s majority ethnic groups will no longer comprise a numerical majority.”

The findings mean that the UK could overtake the United States as the world’s melting pot, with fewer people describing themselves as British or white.

The projection comes after David Cameron sought to reassure voters that the influx of migrants from Bulgaria and Romania will be “nothing like” the levels seen when Poland joined the European Union.

The professor of demography at Oxford made his findings from a study of international population projections, carried out for the academic organisation Migration Observatory.

In the study, minorities are classed as people who also describe themselves in censuses as Irish or another nationality, as well as by their skin colour.

Prof Coleman said migration has become the “primary driver of demographic change”.

According to the data, around a fifth of people in the UK are non-white or non-British. But this is expected to rise to a quarter by 2025, a third by 2040 and reach up to 38 per cent by 2050.

The increase from 2010 to 2050 in the UK – by 22 percentage points – is the highest of the main western countries analysed.

Over the same period, the proportion of non-whites and migrants in Denmark will rise from 10 per cent to 14 per cent.

Declining birth rates among white Britons is another factor.

In England and Wales, 25 per cent of births are to foreign-born mothers, the report said. A similar trend is seen in France and Germany.