An announcement by Schering AG that it former suitor Merck KGaA now holds around 10% of its shares has raised suggestions that a planned 16.5 billion-euro ($21bn) acquisition by Bayer could be under threat.

Bayer said earlier this month that it was extending the deadline for the offer to June 14 after securing just 30% of Schering’s capital, well short of the 75% it needs under the terms of its original offer.

Merck said yesterday it owned around 6% of Schering, but a filing to the Securities & Exchange Commission made by Schering yesterday indicated that its holding was around 10%.

The purported increase sparked speculation that Merck may still be hopeful of dragging Schering to the alter, despite having a 77 euro-per-share takeover offer rejected by the latter after Bayer came forward with its own 86 euro bid. Meanwhile, analysts said the increase in holding could make it harder for Bayer to meet its target stake by the deadline, particularly if Merck refuses to sell its shares to its rival.

Under German law, Bayer cannot make any further changes to the takeover offer, which includes changing the minimum acceptance threshold, the offer price or the acceptance period, without Schering's agreement. But with Schering a willing object of Bayer’s affections there is no suggestion that the transaction will not eventually go ahead.

Bayer's pharma business would be transformed by the acquisition, which would create a unit with 9 billion euros in sales, a broader portfolio in specialty medicines in areas such as cardiovascular, oncology and reproductive health and a bolstered pipeline. it would rank as the 12th-largest drugmaker in the world.