Precious wildlife habitats and sites of special scientific interest (SSSIs) are at risk due to ministers' preoccupation with Brexit, MPs have warned.
Environment Secretary Michael Gove has been accused of "decimating" Natural England, the body that oversees such sites, after it emerged 50 of its staff had been moved to Brexit-related work.
The redeployment means work in some low-priority areas is now on hold.
But Mr Gove insisted monitoring of SSSIs had been taken on by other staff.
The Department of Environment, Food and Rural Affairs (Defra) has one of the largest Brexit-related workloads, given the major changes on the horizon for the UK's farming and fishing industries outside the EU.
In a letter to the Environmental Audit Committee, Mr Gove said his department was hiring 2,000 extra staff to deal with Brexit-related matters and that 1,800 of these were already in post.
But he also revealed that 400 staff had been transferred from other public agencies under the department's aegis, including:
- Natural England
- the Environment Agency
- the Rural Payments Agency
From Natural England alone, 50 staff have been drafted in, on two-year secondments.
Mary Creagh, the Labour MP who chairs the cross-party committee, said this was worrying given the vital statutory work undertaken by the agency in protecting the UK's natural environment.
"Natural England must not become a poor relation to Defra," she said. "Ministers must ensure the valuable work it does to protect biodiversity is given the priority it deserves.
"Preparations for leaving the EU must not get in the way of protecting our treasured natural spaces and iconic British wildlife."
Green Party MP Caroline Lucas said the "raid" on Natural England compounded already deep cutbacks at the agency, whose funding for protecting the UK's 4,126 SSSIs had been cut by 55% since 2010.
She is calling on Mr Gove to reinforce the agency's specialist natural environmental officers at once.
"This letter proves Michael Gove's promise of a green Brexit is pure bluster," she said.
"The government's poaching of Natural England staff for Brexit work is the latest in a string of cutbacks that are decimating the agency that looks after irreplaceable habitats and beautiful landscapes."
Of the 50 staff who have transferred, 13 were previously devoting 50% of their time to working on SSSIs.
Mr Gove said these responsibilities had been passed "to others to absorb", emphasising that his department would continue to fulfil its statutory duties by hiring staff on a temporary basis or promoting within.
"When identifying candidates for secondment, consideration was given to ensuring the potential impact on Natural England's key performance indicators was minimised," he said.
But he added: "Secondees' substantive roles which are not deemed a high priority have been left unfilled and work reallocated or paused for now."
SSSIs are designated on the grounds of their special conservation value, in terms of their wildlife or their geological or topographical characteristics.
Natural England is expected to inspect most sites at least once every six years to check they remain in a "favourable condition" but MPs have warned 47% have not been examined since 2012 in breach of guidelines.