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Canadian firm Valeant Pharmaceuticals said it plans to improve its unsolicited $47bn (£27.9bn) offer to buy US drugmaker Allergan, maker of the Botox anti-wrinkle treatment.

On Monday, Allergan rejected the offer, but Valeant said it will announce a new offer on 28 May.

The firm said it will hold a webcast for Allergan shareholders, thereby bypassing Allergan's board.

Valeant is trying to become the fifth-biggest drugmaker by 2016.

Last year, the firm bought Bausch & Lomb, one of the biggest makers of eye health products.

, Valeant wrote: "As you are all aware, yesterday we received Allergan's letter rejecting Valeant's offer. We note that our offer was rejected without Allergan having had any discussions with Valeant."

"We will not stop our pursuit of this combination until we hear directly from Allergan shareholders that you prefer Allergan's 'stay the course plan' to a combination with Valeant," said Valeant chief executive Michael Pearson in statement.

Allerga rejected the offer because it said it had questions about Valeant's long-term growth as well as its business model.

"The Board has determined that Valeant's proposal creates significant risks and uncertainties for Allergan's stockholders and believes that the Valeant business model is not sustainable," said Allergan board chair and chief executive David E.I. Pyott in .

However, Allergan's largest shareholder - the hedge fund Pershing Square, which is run by activist investor Bill Ackman - had previously said it supported the bid