KOLKATA (AFP) - Police in the Indian city of Kolkata said on Monday they had arrested a local businessman suspected of drugging and raping a 21-year-old Irish charity worker after her birthday party.

The woman filed a police case on Saturday, saying she was assaulted in a semi-conscious state after drinking a cup of tea at the 25-year-old suspect's home following a night out.

"We suspect the accused laced the tea with drugs and raped the Irish woman," police officer Subrata Dey said from the eastern city.

The Irish national, an engineering student who came to India to volunteer with a local charity, has been sent for a medical examination at a state-run hospital.

The accused had befriended the woman at a busy market last week and they decided to party together on Friday night to celebrate her birthday, police say.

A survey by a trade body released earlier this year found that the number of foreign women tourists visiting India had dropped by 35 per cent following several sex attacks that have made global headlines.

The Associated Chambers of Commerce and Industry of India said female tourist arrivals fell after the fatal gang-rape of a 23-year-old Indian student by six men on a bus in New Delhi in December.

The gang-rape brought simmering anger about endemic sex crime to the surface and led to sometimes violent protests in cities across the country.

A Swiss cyclist was gang-raped in central state of Madhya Pradesh in March, while a South Korean tourist was allegedly drugged and raped in the same state in January by the son of the owner of a hotel where she was staying.