Professor Sir John Bell, the scientist who served as Boris Johnson's COVID testing tsar, has quit the investment company which backs start-ups created from Oxford University's academic research.

Sky News has learnt that Prof Bell, who is regarded as one of the world's leading medical scientists, stepped down as a non-executive director of Oxford Science Enterprises (OSE) several weeks ago.

One source close to the company said he had decided to resign, and that the company had sought to persuade him to stay on.

Prof Bell, who serves as Oxford's Regius Professor of Medicine, was among OSE's founding directors.

The timing of his resignation was described by the source as significant, with OSE in the middle of a search for a new chief executive.

OSE, which used to be called Oxford Sciences Innovation, is an investor in dozens of companies, including Vaccitech, which created the biotech platform behind AstraZeneca's COVID-19 vaccine.

It also counts First Light, the nuclear fusion energy start-up, and Animal Dynamics, a deep-tech company, among its portfolio.

In an internal memo announcing Prof Bell's departure from the board and which has been seen by Sky News, OSE said: "It is with great regret that we announce Professor Sir John Bell has decided to step down from the board of the company to focus on other commitments.

"John was a founding director of OSE and has played a vital part in the development of the company since its creation in 2015."

The memo quoted Chris Chambers, OSE chairman, as saying: "John's wise counsel will be sorely missed, and we owe him an enormous debt of gratitude for his support and friendship over the years.

"I am grateful that John has offered ongoing support for OSE, and he will remain close as we continue to grow the portfolio and support the Oxford Science ecosystem."

Professor Bell said in the memo: "My current commitments are such that I do not have sufficient time available to fulfil my duties as a director of OSE and so, reluctantly, following last year's successful fundraise, I have decided that this a good time to retire from the board.

"I have been part of OSE since it was set up, am proud of the work it has done to develop Oxford's ecosystem, and will continue to support OSE in any way I can."

The exit of Alexis Dormandy as chief executive of the Oxford spinouts vehicle came just months after it raised £250m from shareholders in a rights issue.

Two sources told Sky News in December that Mr Dormandy had effectively been forced out of the role.

Mr Dormandy was a prominent recruit to run OSE in 2020, having started his career at Sir Richard Branson's Virgin Group, where he helped to launch Virgin Mobile and Virgin Active.

OSE has a roster of blue-chip group of investors regarded as rivalling any comparable vehicle in the world.

Among its publicly disclosed shareholders are Google Ventures, Sequoia Capital and Temasek Holdings, the Singaporean state fund.

Huawei Technologies, the Chinese telecoms technology giant, is also a shareholder.

The company has endured a turbulent period in terms of management churn, with several chairs and senior executives quitting in a short space of time.

Co-founded by Alex Snow, the well-known City executive, OSE said its search for a successor to Mr Dormandy was continuing "as planned".

Among the other start-ups it has backed or created are Osler, a blood diagnostics venture, and Bibliu, a digital textbooks platform.

Last year, two of its portfolio companies - MiroBio and DJS Antibodies - were sold to global pharmaceutical companies for a combined consideration of $655m.