Drugs giant Pfizer has disclosed details of a £59 billion approach for UK rival AstraZeneca in what would be the biggest foreign takeover of a British firm.
Astra has spurned two advances from the Viagra maker since January but the American company said it remained "confident a combination is capable of being consummated", as it lifted the lid on the deal talks following a week of speculation.
Pfizer admitted it hopes to slash its tax bill as part of the deal, which would see it re-domiciled in Britain.
But Ian Read, chief executive of the Viagra maker, said there were no guarantees for manufacturing jobs staying in the country as he set out details of the approach to Astra.
Scottish-born Mr Read said it would mean one of the biggest pharmaceutical companies in the world being based in the UK.
He also said such an offer would represent a 100 billion US dollar (£59 billion) windfall for the economy - with the proposal to Astra's shareholders comprising a significant cash sum as well as shares in the new firm.
But Pfizer said that under its proposal the new company would retain its head office in New York and be listed on Wall Street.
Mr Read said it was driven by Astra being a good fit for the company, combining to produce efficiencies and delivering earnings growth, opportunities in key drug areas and enhanced presence in developing markets.
Patients would also benefit through the companies' shared commitment to research and development in the fight against diseases such as cancer, it was claimed.
But Mr Read said that the takeover could have seen Pfizer faced with paying 38% tax on earnings from Astra compared with 21% if re-domiciled in the UK.
Chief financial officer Frank D'Amelio said: "We clearly are projecting a lower tax going forward."
Mr Read said Pfizer had "reached out" to the Government today over its proposals, which it set out for the first time in a stock exchange announcement following a week of speculation that has seen the UK firm's share price soar by a quarter.
But he said he could not make any firm commitment about keeping manufacturing in Britain or the future of the new research centre being developed by Astra in Cambridge.
However he added: "We do see the UK as an attractive place to do science and manufacturing. We do believe the UK Government has created the right incentives."
Mr Read said investment in jobs "tends to follow" such incentives.
But Pfizer first faces a major stumbling block in the shape of the AstraZeneca board, which said the initial proposal "very significantly undervalued" the company and its future prospects.
It disclosed that a proposal in January, valuing the company at £58.7 billion, representing a 30% premium to the share price at the time, had been rebuffed, and that it had again been rejected when it tried to revive discussions on Friday.
AstraZeneca admitted that after being contacted initially by Mr Read in November last year, it had agreed to meet its American suitor in New York before the initial deal was rejected.
As well as rejecting Pfizer's valuation, it said it had concerns about a deal structure containing a high proportion of shares as well as risks around re-domiciling Pfizer to the UK for tax purposes.
Astra employs more than 50,000 people around the world, including 6,700 in the UK, but numbers are being reduced as it shifts its headquarters to Cambridge, closing a research centre in Cheshire and offices in London.
The company was itself formed through the merger of Sweden's Astra and Britain's Zeneca in 1999. It is one of the world's biggest pharmaceutical firms and produces a large range of medicines including cancer and diabetes drugs.
Revenues were down 6% to 25.7 billion US dollars (£15.3 billion) last year, reflecting the loss of exclusivity on a number of drugs it had developed as patents ran out.
Pfizer employs more than 70,000 people around the world including 2,500 in the UK and 900 at its UK headquarters in Surrey.
Its revenues of 51.6 billion US dollars (£30.7 billion) for 2013 were also down 6% on the previous year. A takeover of AstraZeneca would dwarf its previous deal to buy rival Wyeth for 68 billion US dollars (£48 billion) in 2009.
Labour's shadow business secretary Chuka Umunna said a takeover should be judged on whether it promoted jobs and growth, protected Britain's research and skills base and guaranteed long-tem investment needed for the UK.
He said: "We want to see Britain's research and development capabilities strengthened. This is crucial for expanding the high-skilled jobs we need to earn our way to higher living standards.
"We also want to see a long-term view taken into account when takeovers happen rather than an exclusive focus on the immediate returns for investors."
Downing Street declined to comment on the proposed deal but a spokesman for the Prime Minister said: "We will always want to work with employers and the business community to maximise prospects for jobs in the UK."