A Japanese pharmaceuticals group has astonished the Tokyo market by taking over Ranbaxy Laboratories, India's largest maker of generic drugs, in a deal worth up to $4.6 billion (£2.3 billion).

Daiichi Sankyo's move on Ranbaxy was described by Tokyo investors as being bold and out of character and it may mark the start of a strategic shift towards emerging markets by Japan's pharmaceutical industry.

The agreed offer for 50.1 per cent of Ranbaxy's shares is a response to a worsening profit crisis in Japan's drugs industry. The remedy may lie in emerging markets, analysts said.

The transaction marries two classes of company that traditionally have been foes.

Makers of generic drugs produce ultra-cheap copies of drugs invented by developers such as Daiichi.

Ranbaxy has been especially aggressive in challenging patents that protect blockbuster medicines worth billions of pounds in annual revenues.

The merged group, which will have a market value of about $30 billion, will combine Daiichi's research expertise with Ranbaxy's low-cost factories and global distribution network, the companies said.

The takeover is thought to be the largest of an Indian listed group by a foreign buyer.

Takashi Shoda, chief executive of Daiichi Sankyo, said that the deal would “complement our strong presence in innovation with a new, strong presence in the fast-growing business of non-proprietary pharmaceuticals”.

Countries with ageing populations, such as Japan, where generic drugs at present comprise only 5 per cent of the market, are expected to encourage use of generic drugs to cut healthcare costs.

Daiichi has agreed to buy the 34.8per cent stake in Ranbaxy held by the founding Singh family at 737 rupees a share, a 31 per cent premium to Tuesday's closing price.

Under Indian takeover rules Daiichi must make a tender offer for a further 20 per cent.

Malvinder Mohan Singh, Ranbaxy's chief executive, said that the sale would create “a new powerhouse ... spanning the entire pharmaceutical spectrum”.

Making only generics, which typically sell at a 97 per cent discount to their patented templates, was not a sustainable business model, Mr Singh said.

Cost-cutting and new drug pipelines do not appear to have played a role in Daiichi's decision, and although investors queried what appeared to be a deal with no synergies, the move by Daiichi will give the group access to generic drugs markets in 50 countries.

To try to cut costs, the Japanese Health Ministry has sought to persuade doctors and patients that generic drugs are as good as their branded equivalents, but it has met with failure.

Even changing prescription forms to make generic drugs the default recommendation of doctors has been unsuccessful.

Part of the problem, state officials say, is that generic drug companies in Japan are small and doctors do not trust them.

By effectively rebranding Ranbaxy generics under the well-known name of Daiichi Sankyo, this may change.

Mr Singh, who has made a series of acquisitions in recent years, said that he would stay Ranbaxy's chief executive.

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