New Delhi: Shares in Ranbaxy Laboratories Ltd fell 10 per cent on Monday, their biggest drop in 7 years, on concerns a US probe against the drug maker may hit sales in a market responsible for a quarter of its sales.

A Ranbaxy spokesman said on Sunday the US Food and Drug Administration had filed a motion in a US court seeking documents from Ranbaxy, India's top drug maker by sales, as part of an ongoing investigation.

Indian newspapers reported US authorities have been carrying out an investigation related to sub-standard drug sales in the United States.

Mamoru Tsunoda, spokesman for the Japanese drug maker, said Daiichi Sankyo knew the FDA was conducting an inspection but had no idea of the latest development.

"We would like to continue our work until the deal is closed while also keeping close eyes on what's going on between Ranbaxy and FDA," he said.

Shares in Ranbaxy fell as much as 10.5 per cent in the morning and were down 8.8 per cent at Rs 484.50 in a flat broader market. It was the stock's biggest daily drop since 2001.

In Tokyo, shares in Daiichi Sankyo fell 5.5 per cent.

Some analysts said the probe is unlikely to affect the deal.