Microsoft founder Bill Gates declined an offer to serve as Donald Trump's science advisor, telling the US President the role was "not a good use of my time".

In a new interview, Gates disclosed that Trump offered him the long-vacant White House science adviser position during a conversation in the Oval Office last month.

Without explaining his reasons, Gates said he turned down the offer and admitted he does not know if the US President was serious, according to the Daily Mail.

He told health sector website Stat News: "I mentioned, 'Hey, maybe we should have a science advisor'. He said: Did I want to be the science advisor?

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"I didn't put him to the test, whether that was a serious thing or not.

"He probably himself didn't know if he was serious. It was a friendly thing. He was being friendly."

Gates said he told the President the role was "not a good use of my time".

Trump has taken longer than any other modern president to fill the science adviser position, which has remained vacant since the departure of John Holdren, who left at the end of the Obama administration in January 2017.

The White House's de facto senior science adviser is 31-year-old Michael Kratsios, a political scientist with no formal scientific training.

While Gates is not a scientist by trade, he co-runs the Bill and Melinda Gates Foundation, which directs a wealth of climate and infectious disease research.

Last month it was revealed the philanthropist was backing a start-up that plans to provide "live and unfiltered" high definition footage of Earth from space.

The company, EarthNow, says it will launch 500 satellites to cover Earth's atmosphere in real-time video surveillance.

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As well as Gates, SoftBank CEO Masayoshi Son and aerospace giant Airbus have also backed the project.

The price of the project could run to US$1 billion ($1.4b).

Potential applications include tracking illegal fishing, monitoring the weather and tracking natural migrations globally.

Gates has previously said improved surveillance is vital to monitor malaria-carrying mosquitoes in vulnerable nations.