AstraZeneca said on Monday that an informal merger offer from USdrugs giant Pfizer valuing its British rival at almost $100 billion "very significantly"undervalues the company."The board ... concluded that the proposal very significantly undervaluedAstraZeneca and its prospects," said a statement issued after Pfizer earlier on Monday had revealed its bid plans.The statement added that "the board remains confident in the ongoing execution ofAstraZeneca's strategy as an independent company and that its successful deliverywill create significant value for shareholders".Pfizer had said that it remained interested by a blockbuster merger withAstraZeneca that would fuel cancer research, after its informal offer was rejected inJanuary.The offer of nearly $100 billion is equivalent to about 72 billion euros.