AstraZeneca, Britain's second-largest pharmaceutical company, is braced for a renewed takeover offer from its US rival Pfizer as its fight to remain independent enters a critical seven days. Pfizer is widely expected to raise its cash and shares offer from £50 a share to at least £53, with a larger cash element, in an attempt to kickstart talks. This would value the Anglo-Swedish drugmaker at nearly £67bn, compared with the 2 May proposal of £63bn – of which less than a third was cash.

Investors and analysts believe a new offer is imminent after a tour of AstraZeneca shareholders by Pfizer chief executive Ian Read last week left the impression among City institutions that the US group is strongly committed to securing a takeover.

The New York-based Viagra maker has until 5pm next Monday to lodge a formal offer and meet a "put up or shut up" deadline imposed by the Takeover Panel, or walk away for six months.

While a £53 a share offer would increase the pressure on the AstraZeneca board to start talks, it could take a bid of £55 a share to get a deal done, according to some analysts and shareholders. AstraZeneca's shares ended the week trading at £48.23.

AstraZeneca boss Pascal Soriot, who has so far refused to negotiate, said last week that the current offer was "really far from what we think the value of the company is". AstraZeneca has spent the last two weeks talking up its drug pipeline, in particular a new generation of cancer treatments that target the body's immune system.

A hostile bid, which would see Pfizer bypass the board and appeal directly to AstraZeneca's shareholders, appears unlikely given the complexity of the proposed deal.

Pfizer wants to move its tax domicile to Britain to cut its tax bill, which has sparked controversy in Britain and among US politicians. The company has also worried British politicians and trade unions with an admission that it would cut global research and development spending and jobs.

Pfizer has declared its preference for a recommended offer and told AstraZeneca shareholders last week that talks with the board and access to the firm's books would help deliver "optimal deal terms and structure".

Soriot told the Guardian last week that AstraZeneca's reputation could be damaged by Pfizer's tax avoidance plans and the proposed break-up of the business. AstraZeneca's assets would be split between Pfizer's three new divisions. But he also acknowledged his duty to shareholders, saying the board would have to consider a credible bid.

Pascal Soriot, chief executive of AstraZeneca, told MPs that life-saving drugs could be delayed by a Pfizer takeover. Photograph: Facundo Arrizabelaga/EPA

Two of AstraZeneca's top 10 shareholders, Sweden's Investor AB, with a 4.1% stake, and Aberdeen Asset Management, with 2.4%, have publicly backed the board's rejection of Pfizer's advances.

The influential fund manager Neil Woodford, whose new venture does not yet own shares in AstraZeneca, has also backed the Anglo-Swedish firm. But other big investors want it to start negotiations. Read is thought to have met a dozen institutional shareholders in AstraZeneca last week, including two hedge funds.

Pfizer is under pressure from the UK government to extend from five years to 10 years its commitment to keep 20% of the combined research and development workforce in Britain.

British civil servants have begun exploratory talks with the EU about amending the terms of the public interest test to include continued investment in research and design as grounds on which the government could threaten to block the takeover.

Vince Cable's business department is also interested in using the test to obtain stronger commitments from Pfizer on jobs and R&D. The Labour party has also said it would subject any deal to an amended public interest test if it won the general election.

Any takeover would be vetted on competition grounds by the European commission and regulatory authorities in the US and China, which competition lawyers say could take more than a year.

AstraZeneca's chairman, Leif Johansson, reiterated over the weekend that the price was too low and pointed to regulatory uncertainties, particularly in China. "We identified a big execution risk," he told the Sunday Times. "It could be over a year or more before the transaction happens."

Sir Tom McKillop, who ran AstraZeneca from 1999 until 2006and is a former president of the Science Council, became the latest senior industry figure to voice serious concerns about the takeover. Writing in the Sunday Times, he urged the firm's directors and shareholders to "protect the 'soul' of this great business". McKillop oversaw the 1999 merger between Sweden's Astra and Britain's Zeneca, which he described as a "genuine merger of equals" that "allowed increased investment in R&D, not the reduction that Pfizer proposes".

McKillop said Pfizer and AstraZeneca, by contrast, had "different visions and conflicting cultures; it is unlikely that they will make a good fit together". He concluded by telling AstraZeneca's directors that they must remember that their fiduciary duties extend not just to shareholders, but to the wider society – "including the millions of patients hoping for new treatments that can only come from a commitment to research."

The Swedish trade unions Unionen, Akademikerföreningen and IF Metall, representing workers at AstraZenca's sites in Sweden, echoed concerns voiced by British unions Unite and GMB. They said: "Pfizer's historical track record on mergers and acquisitions is quite scary. Takeover after takeover has been followed by redundancies and closures all over the world."

Meanwhile, the US drugstore chain Walgreens is reportedly preparing to take full control of Britain's biggest chemist, Alliance Boots, in a £10.5bn deal. It is the latest attempt by a US company to redomicile to Britain to save billions in tax. Walgreens already owns 45% of Boots and has an option to buy the remaining 55% from next February with a six-month window.