UK pharmaceutical major AstraZeneca has smacked down Pfizer's $100bn takeover bid after claiming that discussions with its US rival proved unattractive and "undervalued" the company.

Britain's second-biggest drugmaker said in a statement that it received an initial contact from Ian Read, chairman and CEO of Pfizer, on 25 November 2013, asking for discussions regarding a merger of the companies.

AstraZeneca agreed to an exploratory meeting on 5 January in New York. At this meeting, Pfizer made a preliminary proposal to acquire AstraZeneca shares at £46.61 a piece in a cash and stock deal, the company said.

The proposal also involved a new US-listed and headquartered holding company.

"The Board of AstraZeneca concluded that the Proposal very significantly undervalued AstraZeneca and its prospects," the company said.

It also questioned the transaction structure and raised concerns regarding the proposed "inversion structure as Pfizer would redomicile to the UK for tax purposes".

Following the meeting, AstraZeneca informed Pfizer on 12 January of its rejection of the proposal. Subsequently, Pfizer notified AstraZeneca on 15 January that it was no longer considering the proposal, according to the statement.

AstraZeneca concluded that lacking "a specific and attractive proposal, it was not appropriate to engage in discussions with Pfizer."

"The Board remains confident in the ongoing execution of AstraZeneca's strategy as an independent company and that its successful delivery will create significant value for shareholders," the company added.

Earlier, the company's shares rose more than 14%, after Pfizer confirmed it is still pursuing a merger deal, roughly valued at £60bn (€73bn, $100bn).

Pfizer confirmed that it had unsuccessfully held discussions with AstraZeneca for a potential merger with the firm on two occasions, and is seeking alternatives with respect to the company.

"As in its previous proposal, Pfizer is considering a possible transaction in which AstraZeneca shareholders would receive a significant premium for their AstraZeneca shares, to be paid in a combination of cash and shares in the combined entity," the company said.

AstraZeneca shares are trading up 14.12% as at 9:16 am GMT.

Meanwhile, AstraZeneca has cautioned its shareholders that it has no certainty that an offer will be made, and advised them to take no action.

It added that it remains committed to executing the strategy announced in March 2013, that focuses on re-establishing scientific leadership and returning to growth.