Defra's long-serving chief scientific adviser, Professor Ian Boyd, has announced that he is leaving the department in the summer.
Boyd – a marine ecologist who was in January awarded the prestigious Polar Medal for his years of work with the British Antarctic Survey – joined Defra in 2012 on a three-year contract, which was later extended.
But the department's top scientist announced on Monday that he would not be continuing in post when the role comes up for renewal later this year.
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He posted on Twitter: "I've made the tough decision to not renew my role as Defra CSA in August."
A Defra spokesperson separately confirmed the move to CSW, saying that Boyd's departure had been discussed for several weeks and marked the "natural end" of his fixed term.
No detail has yet been given on process for replacing Boyd, but the outgoing chief scientist tweeted: "Business as usual in the meantime while I help find a successor."
Boyd, who has kept a lively personal blog during his time as Defra's CSA, last year wrote a piece paying tribute to the "vital work being done by government scientists", describing them as "a modest bunch of people who rarely talk openly about the great work they do".
At Defra, Boyd has worked under four different environment secretaries, providing advice to ministers on a wide range of policies, including contentious areas such as the badger cull and the use of controversial neonicotinoid pesticides.
The latter saw Boyd clash with environmentalist George Monbiot in 2013, with the Guardian columnist accusing him of seeking to "shut down programmes that produce unwelcome findings" and "establish a tinpot dictatorship".
But Boyd hit back, saying that it was not the job of a government scientist to "make politicians' decisions for them".
"If scientists start to say one or other option is right or wrong, they begin to take the position of politicians and devalue the scientific evidence they claim to present," he argued.