Mumbai: Shares of Ranbaxy Laboratories, which is set to be taken over by Japanese drug firm Daiichi Sankyo in a $4.6 billion deal, fell almost 10 per cent today, following reports that the generic drugs major is under investigations in the US for sale of sub-standard drugs.

The Ranbaxy stock suffered its largest one-day loss in almost seven years on the Bombay Stock Exchange on concerns that the ongoing probe may hit sales in the US market which accounts for a quarter of the company's sales.

Reports, quoting sources, said the US Food and Drug Administration moved the court saying the quality of drugs made at one of Ranbaxy's domestic plants was poor.

A spokesperson for Ranbaxy, India's top drug maker by sales, said the FDA had filed a motion in a US court seeking documents from Ranbaxy, as part of an ongoing investigation, but denied initiation of any legal proceedings against the company.

Reports quoting the the Star-Ledger said the FDA filed the the complaint in the US district court of Maryland on 3 July after it found that the Ranbaxy drugs had too little or too much active ingredient or lacked the labelled shelf life and didn't meet US FDA standards.

''Ranbaxy strongly denies these allegations contained in the motion," said a company spokesperson.

A spokesman for Japan's Daiichi Sankyo, which is set to acquire a majority stake in Ranbaxy, said it knew of the FDA inspection and that it was keenly watching the developments.

''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," Daiichi spokesman Mamoru Tsunoda said.

Ranbaxy shares fell as much as 10.5 per cent and were down 8.8 per cent at Rs484.50 in a flat market.

Daiichi Sankyo shares fell 5.5 per cent in the Tokyo market.

Japan's Daiichi Sankyo, which acquired 34.8 per cent in the company from its promoter directors, is making an open offer for a further 20 per cent, in a deal worth a total of $4.6 billion.