An international team of scientists has uncovered striking evidence that mass conversions to Catholicism by Sephardic Jews and Muslims took place in the 15th and 16th centuries in Spain and Portugal.

The discovery shines new light on one of the bitterest episodes in the history of the Iberian Peninsula when the region entered a period of terrible religious oppression. The Moors, who at first adopted a policy of religious tolerance when they conquered Spain, later introduced laws that forced Christians and Jews to convert to Islam. Then, after Christians had achieved key victories over the Moors, they expelled all Jews and Muslims who would not convert to their religion.

In the past, it was assumed these two acts triggered two separate, massive waves of expulsions. The discovery controversially challenges this belief. The new study, reported in the American Journal of Human Genetics last week, indicates that large numbers of people, particularly Spanish and Portuguese Jews wishing to stay in Spain, converted to Catholicism rather than lose their homes and livelihoods.

The research was carried out by a team led by Professor Mark Jobling of Leicester University and Francesc Calafell of the Pompeu Fabra University in Barcelona. They found that 20 per cent of men in Spain and Portugal today still have distinctive Sephardic Jewish ancestry while 11 per cent have DNA that reflects Moorish ancestors.

'The Jewish link was particularly surprising,' said Calafell. 'We had certainly not expected it.' The evidence from DNA studies indicates that rather than practise their religious beliefs, many Spaniards and Portuguese chose to give them up in order to avoid expulsion. Their decision to do so is revealed through the genetic make-up of modern Spanish Catholic men who were often unaware of their ancestry.

The team's study was based on analyses of Y chromosomes of Sephardic Jewish communities in places where Jews migrated after being expelled from Spain in the 1490s. In addition, the scientists characterised the Y chromosomes of the Arab and Berber army that invaded Spain in AD 711 from data on people living in Morocco and the Western Sahara.

Then they compared these Y chromosomes with more than a thousand men currently living in Spain and Portugal. Use of the Y chromosome, which confers masculinity, was crucial because it remains unchanged as it is passed from father to son. In this way, the researchers were able to determine if a man's Y chromosome came from a Jewish or Moorish predecessor or another source.