* Takeda mulling a bid for Shire that may top $40 bln
* Bid could spark takeover battle in pharma industry
* Takeda shares sink, Shire's surged (Adds details, background)
TOKYO, March 29 (Reuters) - Takeda Pharmaceutical shares sank to their lowest level in almost a year on Thursday after Japan's largest drugmaker said it was considering a bid for London-listed Shire that could top $40 billion.
Shares in Takeda fell more than 6 percent at one point in early morning trade, the lowest since last April, sharply underperforming the broader Tokyo market, which was around 1 percent.
Takeda said on Wednesday it was at a "preliminary and exploratory stage" of considering a bid for the rare disease specialist, adding that it had not approached Shire's board. Shire said it noted Takeda's statement, and confirmed it had not received an approach.
Takeda's potential bid for Shire, most of whose employees are based in North America, immediately stoked expectations for another takeover battle in the deal-hungry pharma industry.
Shire's shares ended 15.7 percent higher, valuing the group at about 32 billion pounds ($45 billion).
Shire sells treatments for rare diseases and attention deficit disorder. Takeda said buying it could create a global biopharmaceutical leader, boosting its position in the United States and in the fields of oncology, gastrointestinal diseases and neuroscience.
The drugs industry has seen a surge in dealmaking as large players look for promising assets to improve their product pipelines.
In recent months, France's Sanofi agreed to buy U.S haemophilia specialist Bioverativ for $11.6 billion and Belgium's Ablynx for 3.9 billion euro ($4.8 billion). Before that, U.S.-based Celgene bought cancer specialist Juno Therapeutics.
($1 = 0.8125 euros) (Reporting by Minami Funakoshi; Editing by Chang-Ran Kim and Stephen Coates)