The millionaire founder of Oxford vaccine giant PowderJect is to leave the company if a takeover by an American rival is successful.

PowderJect chief executive Paul Drayson, at the launch of the Oxford Children's Hospital campaign, of which he is chairman

Dr Paul Drayson, who started the company using technology developed by his father-in-law Prof Brian Bellhouse at Oxford University 11 years ago, made the announcement after it was revealed he was backing a £542m offer from US group Chiron.

The 43-year-old, his wife Elspeth, who helped him set up and run the firm, and Prof Bellhouse stand to make £108.4m from the sale of their 20 per cent stake.

About 400 workers at the headquarters on the Oxford Science Park now face an uncertain future while shareholders vote on the proposed deal. But Dr Drayson said he is confident the research and development carried out there is one of the company's major assets.

He said: "This is all about growth and the prospects for employees and opportunities for investment are enhanced.

"I don't think it is possible for me to make commitments, but I can see opportunities here in Oxford -- it is an integral part of the business and the research and development is fundamental."

Should the offer be accepted, it will mean Chiron chief executive John Lambert will become chief executive of the combined company.

Dr Drayson refused to become emotional about the decision to sell the business, which he has developed from a start-up company to a business with revenues of £158m.

He said: "I am 43 and don't see myself sitting on a beach. I want to be busy doing something -- what that something is I really don't know at the moment.

"It is really important for emotions not to come into this and chief executives must do the right thing for the business, shareholders and employees."

Dr Drayson, who has a reputation for being a workaholic, said he had invested his "heart and soul" in the firm, which pioneered "needle-free" injection technology devised by Prof Bellhouse.

He has pledged the family's stake in the company to Chiron and said Prof Bellhouse had backed the decision to sell. Chiron needs the approval of a majority of PowderJect shareholders for the takeover to go through.

If it goes ahead, it will be the second Oxford University spin-out firm to be bought out in a month, after Oxford Glycosciences was taken over by Slough-based rival Celltech.

That deal is likely to lead to job losses among the 300 staff at Milton Park, near Abingdon.

Last year, PowderJect made headlines after it was awarded a controversial £32m contract to supply the Government with 20m smallpox vaccines, intended for use in case of bioterrorist attack, and it emerged that Dr Drayson was a Labour Party donor.

This is Chiron's second attempt to buy the firm. PowderJect rejected approaches from both the US firm and pharmaceutical giant GlaxoSmithKline, which owns 5.4 per cent of PowderJect's shares, in November because the offers were considered too low. Chiron has now increased its bid.

A Chiron statement said: "We look forward to working with the people of PowderJect to create a stronger global player in the vaccines business."