Lord Sainsbury today announced his retirement as science minister after eight years in the post, citing personal rather than political reasons.
The former businessman, the longest-serving Government minister after the Prime Minister and Chancellor, said this was the "right moment" to stand down.
The decision was not linked to the current "cash for peerages" investigation, insisted the former chairman of the supermarket chain.
The 66-year-old, who stood down from his ministerial job immediately, said he had been interviewed by police in the summer as a "witness", adding that he did not believe the investigation would lead to anything.
"I am not directly involved in whether peerages were offered for cash, I am not the party's fundraiser and I have been a peer for nine years, so no one is suggesting I was in the business of buying a peerage."
Lord Sainsbury, who has previously made donations and loans to Labour, became a peer after the party was returned to power in 1997 and was appointed science minister in the summer of 1998.
Lord Sainsbury said he would now concentrate on his charitable work and would continue sitting in the House of Lords.
He has also been asked to carry out a review of science and innovation policies across government between now and next summer.
"I am proud of what we have achieved in terms of funding properly our world-class scientists and engineers, in improving the rate of innovation and knowledge transfer and in putting science and innovation at the centre of policy-making," he said.
Tony Blair led a series of tributes to Lord Sainsbury, describing him as a valued colleague who had been a "huge asset" to the Government and the country.
"David has worked tirelessly to push the case for science within Government. It is in no small part down to him that this country's science base is in better shape than for decades, something of vital importance for this country's future.
"His contribution to Government will have a huge impact in future decades and we owe him a real debt of gratitude."
Lord Sainsbury said he only planned to stay as a minister for three or four years, but that had stretched to eight years.
He thought about stepping down in the summer but decided to stay so he could steer the Companies Bill through the Lords.
"I think I have achieved most of what I can achieve as a minister and I will now go back to pick up by business and charitable activities."
Lord Sainsbury remained a major shareholder in the supermarket firm during his time as a minister although his shares were placed in a blind trust, where they will remain for the next three months.
He said he had no intention of returning to an active role at the firm.
There was a series of tributes to Lord Sainsbury from the world of science and medical research.
Dr Mark Walport, director of the Wellcome Trust, said: "David Sainsbury has been good for science and good for innovation in the UK. He has been an outstanding science minister and shown extraordinary passion and commitment to his portfolio."
Colin Blakemore, chief executive of the Medical Research Council, said: "Lord Sainsbury's long and successful period as science minister has been characterised by his irrepressible love of science. He will be remembered fondly as the scientists' minister of science.
"Lord Sainsbury has led British science through a period of increased funding and remarkable achievement, raising even further its influence and impact throughout the world.
"He is hugely respected by researchers for his courage in defending science through a series of challenging public debates - particularly on stem cells and the use of animals in research."
Philip Greenish, chief executive of the Royal Academy of Engineering, said Lord Sainsbury had been an "outstanding" minister, adding: "His belief in and support for UK engineering and for high quality science and engineering education have been inspirational over many years.
"We owe him a great debt of gratitude for his role in transforming attitudes and in creating recognition of the crucial importance of science and engineering to the nation's future."
Baroness Greenfield, director of the Royal Institution, said: "Lord Sainsbury has championed science in all its aspects. He will be particularly remembered for highlighting the need for the private sector to link up with research in universities, which has enabled us to leverage the basic science into applications that can offer benefits to society."Reuse content