The reserves and protected places that are the “jewels in the crown” of English nature cannot be managed properly because of budget cuts, Tony Juniper, the chair of Natural England, has said.

The budget for the government’s conservation watchdog has been slashed in half over five years, leaving it “massively depleted”, according to Juniper, the influential former Friends of the Earth campaigner whom the environment secretary, Michael Gove, appointed earlier this year.

Sites of special scientific interest (SSSIs) cannot be monitored to ensure their wildlife riches are maintained, and on national nature reserves Natural England can only afford to ensure basic health and safety for visitors, he said.

Tony Juniper became Natural England’s chair by appointment of Michael Gove. Photograph: Roberto Ricciuti/Getty Images

“I’ve inherited an organisation that is depleted, massively depleted,” Juniper told the Guardian in his first national newspaper interview since taking the job. “On a whole range of subjects, we cannot do what society expects of us.

“For example, all we’re able to spend on the management of the national nature reserve estate is for health and safety so visitors don’t hurt themselves.

“We’ve got no monitoring capacity on the SSSIs. Our ability to give advice to planning applications and our works on landscapes, national parks and areas of outstanding natural beauty is cut now to pretty much nothing.”

Juniper, visiting Halvergate Marshes in Norfolk, an SSSI and the second largest block of freshwater marshland in Britain, said he had been given no guarantees over budget increases or even an end to cuts so Natural England can revive wildlife and use natural landscapes to help tackle the climate emergency. But he said he had taken the job because he wanted to “reinvigorate the official nature conservation effort in this country”.

Juniper said budget cuts left the watchdog vulnerable to legal challenges. The WildJustice group led by Mark Avery, Ruth Tingay and Chris Packham successfully forced Natural England to scrap the “general licence” that previously allowed landowners to freely kill certain bird species such as crows and woodpigeons.

WildJustice has launched a fresh legal challenge against new temporary general licences, and Juniper invited Avery, Packham and Tingay to meet him.

“They are good friends of mine and I’d be pleased to talk to them about this or anything else, but the involvement of lawyers makes that more difficult,” he said. “I would hope that conservationists could be working together to try to solve these problems in a less time-consuming and confrontational manner because these legal actions do use up a vast amount of resources.”

Juniper appeared to accept that Gove was unlikely to remain environment secretary much longer.

He said: “Michael Gove has been an incredibly energetic, dedicated and effective secretary of state for the environment and it’s very rare we get those. The last one who made that kind of impact was John Gummer back in the early 90s. As was the case with John Gummer we were very surprised how the brief became so passionately owned – Michael Gove has done that and the conservation community has a lot to thank him for, for putting these issues back on the map.”

Juniper said he hoped Gove’s successor would take heed of the public mood, as outlined by recent Extinction Rebellion protests and the school climate strikes. He said: “I would hope no matter what the personal views of the new secretary of state they will come to the role recognising that the voters these days really want delivery on this stuff.”

A spokesperson for the Department for Environment, Food and Rural Affairs said: “Natural England’s work is vital for protecting and enhancing the nation’s natural environment. We have worked closely with Natural England to settle their budget for the coming year.”