Pfizer has sought to dispel fears that its proposed £63bn takeover of AstraZeneca would lead to big cuts in British drug research, a day after Britain's second-largest drugmaker touted its 10-year growth strategy as an independent company.

The news came as the Anglo-Swedish company's fourth-largest shareholder came out in support of the board's rejection of the approach. Sweden's Investor AB, which owns 4.1% of AstraZeneca, said it was "backing the board" of the company – the biggest investor so far to publicly back the drugmaker's battle to stay independent.

An infographic (pdf) published by Pfizer hailed the benefits of a tie-up, with 82 products in the pipeline, and carried a quote from the former GlaxoSmithKline boss Sir Richard Sykes. "Pfizer are serious and they've got a lot of money to spend. They'll need first-class people doing first-class research," it said.

During prime minister's questions, David Cameron rejected claims that he was "cheerleading" for Pfizer, saying he was "fighting for British jobs".

"The commitments that have been made so far are encouraging," he said, referring to pledges from the US firm to complete AstraZeneca's planned scientific campus in Cambridge, to base 20% of the new company's research and development workforce in the UK and to manufacture more here.

But Cameron added:"I'm not satisfied, I want more, but the way to get more is to engage."

Sweden's finance minister, Anders Borg, said Pfizer failed to honour promises made on research jobs when it bought Swedish drugmaker Pharmacia in 2002. He told BBC Radio 4: "Our experience shows that their track record is not very convincing and I think one should take these kind of promises not only with a pinch of salt but a sack full of salt."

The backlash against Pfizer's takeover approach is growing, with the former City minister Lord Myners saying Cameron was ill advised to get drawn into becoming a "cheerleader" for the proposal.

Myners also told ITV that it is "simply not acceptable" to say the tie-up is not a matter of public interest and cannot simply be left to shareholders.

AstraZeneca's chief medical officer, Briggs Morrison, who spent five years at Pfizer, told reporters of his concerns over the commercial culture at the maker of the Viagra and cholesterol drug Lipitor.

AstraZeneca's chief executive, Pascal Soriot, brought a new focus on science when he took over from David Brennan, a former salesman, 18 months ago.

"We are trying to create an organisation where the whole organisation is focused on science," Morrison said. "I didn't have the feeling that every part of [Pfizer] was science based."

It also emerged that two of Cameron's senior advisers have been paid by Pfizer in the past – Mark Textor, a strategist hired by the Conservative party who has lobbied on behalf of the drugmaker, and Gabby Bertin, now the No 10 press secretary, who was paid £25,000 by Pfizer to work as a researcher for the former defence secretary Liam Fox's contentious Atlantic Bridge charity.

The Unite union says AstraZeneca's 6,700-strong UK workforce has a right to know what links the Tory government has to Pfizer.

Unite's assistant general secretary, Tony Burke, said: "We want to know, and we are sure that the public will want to know, what is the extent of the relationship between the government, Cameron's paid strategists and Pfizer?

He added: "To date the government has been banging the drum for a company that appears to want to put UK jobs and R&D at risk so it can reduce its taxes. The Pfizer bid for AstraZeneca goes against the grain of the government's so-called strategy for advanced manufacturing and the national interest."