(Reuters) - Wellcome Trust, Britain's biggest medical research foundation said it had "major concerns" about U.S. drug major Pfizer's 63 billion pound offer for AstraZeneca , the Financial Times reported. In a private letter to UK chancellor of Exchequer George Osborne, the trust raised doubts about Pfizer's commitment to investment in Britain and said AstraZeneca was critical to Britain's science base. "Pfizer's past acquisitions of major pharmaceutical companies have led to a substantial reduction in R&D activity, which we are concerned could be replicated in this instance," Wellcome Trust Chairman Sir William Castell was quoted as saying by the British newspaper. Wellcome Trust became the latest in a series of vocal opponents to the potential merger, which would create the world's largest drugmaker. Pfizer is said to be considering raising its offer after AstraZeneca rejected its previous offer of $106 billion. The value of that cash-and-stock offer has since slipped because of a fall in Pfizer shares following weak quarterly results. The deal faces political scrutiny as pressure builds on Prime Minister David Cameron to protect Britain's jobs and research base should it go through. Wellcome Trust spends more than 750 million pounds a year on biomedical research, according to the FT. ($1 = 0.5899 British Pounds) (Reporting by Tasim Zahid in Bangalore; editing by Jane Baird)