The shrubbery rustles and shakes, then Brian May falls out of the rhododendrons, dusts himself down and stumbles towards five fox cubs at play in a clearing. In the landscaped gardens of his historic home in the Surrey hills, the Queen guitarist looks every inch the semi-retired rock star: huge curly hair on gangly frame, black trousers, immaculate white Pumas and a dangerously unbuttoned white shirt.

May has a plethora of projects to promote – every Queen album is being digitally remastered to celebrate the band's 40th anniversary this year; this month he will tour the country with Kerry Ellis, who starred in his musical We Will Rock You, and with whom he has written a new album; he has just played guitar on a new Lady Gaga track; and a documentary he has made about the history of 3D will be broadcast on Sky. But instead, the 63-year-old musician and astrophysicist is crooning softly to the fox cub he has clutched to his chest.

In the state-of-the-art animal rescue centre May has built in his garden, the guitarist behind Britain's biggest-selling album (Queen's Greatest Hits) is currently nursing back to health 140 hedgehogs and half-a-dozen abandoned fox cubs. He is also fast becoming the public face of the campaign to stop a cull of badgers proposed by the authorities to answer the concerns of farmers who are convinced the animals are infecting cattle with bovine TB.

Multimillionaire rock legends often dabble in fast cars, metal detecting or saving rainforests. Is May worried his anti-cull campaign will be dismissed as just another rock star hobby? "Hobby, hmmm," he murmurs, treating the word with a quiet disgust. "There aren't many people from rock music or entertainment who put the time in that I have . . . this has become a huge part of my life. I don't care what people say. I'm not doing it to make money. I'm not doing it because I want to be famous. Even if it was a 'hobby', why would I have that hobby? It could only be because I care about animals. This concerns us all. It's not just something that concerns farmers."

May, who simultaneously channels the ethereal otherworldliness of a long-term resident of planet rock with the precision of a professional scientist, came to this passion through blogging. He began his "soapbox" before blogging was fashionable and although gentle and softly-spoken in person, he lets off steam online: his rants on subjects such as a review of Mika's album by the Guardian's Alexis Petridis have led him to be dubbed the world's grumpiest rock star. His intention, he says, is to discuss important issues "so if you want to call it being grumpy, yeah, but it's being concerned and trying to raise awareness of things which need to be fixed". There are "joyful" posts about music, astronomy and 3D on the soapbox, but May is most profoundly anguished about cruelty towards animals. The blog "changed my life completely because it's a two-way communication", he says. "I started talking about animals with people on the soapbox."

May has never been an ordinary rock star. Helped by his father, he built his own guitar, Red Special, as a child. He abandoned his PhD on zodiacal light for fame with Queen but returned four years ago and completed it. He is a great friend of astronomer Sir Patrick Moore, with whom he has written Bang!, an accessible history of the universe.

His love of animals has been less well documented but "it's always been there", says May a little defensively, highlighting his Queen song White Man as an example. He promised himself "if there was ever an opportunity in my life to make a difference for animals I would take it". That opportunity came when someone contacted him online about a proposed cull of hedgehogs on the Scottish island of Uist. May was "aghast" and successfully fought to have the hedgehogs transported to the mainland instead. "When David Cameron started saying that if he got into power he would try to repeal the hunting act, my ears really pricked up and I thought if I can make a difference with hedgehogs maybe I can alert people to the possibility of going back into what I regard as something very barbaric," he says.

May, who is married to the actor Anita Dobson, and has three children from his first marriage, and three grandchildren, has set up a charity, Save Me, which rehabilitates injured wild animals, and is campaigning against the badger cull and any repeal of the hunting ban. Dobson is supportive but does not get involved. "Everybody has to work on their own passions," says May very quietly.

"So many people have said to me, foxes are just like rats, who gives a shit? And then you say, so a rat isn't worthy of some consideration? Rats are so like human beings, it's frightening. And it's human beings who are relegating them to 'vermin'. It gives people the feeling that they don't have to treat animals with any kind of consideration, and to me that's wrong."

May, who has long been an "imperfect vegetarian" but does not now eat meat or fish, "certainly wouldn't" engage in direct action, such as releasing animals from scientific laboratories or mink from farms but thinks that animal rights has become a dirty word because its image has been "deliberately manipulated" by opponents who incorrectly characterise hunt saboteurs as violent. Hunting and killing animals for pleasure "boggles my mind", he says. "I don't understand why people are like that. I wrestle with that the whole time. It's a sickness. I don't think a healthy human being needs to be killing or causing pain to be happy. Why should that be? A decent life and a decent death – that's what I ask for myself, and that's what I would ask of any creature." Ultimately, he believes animals have rights – both moral and legal – and wants to challenge "the mentality that says human beings are the only creatures on this planet who matter".

May happily plays with the fox cub he has named Caroline Spelman, after the environment secretary, but does not want to be caricatured as sentimental about animals.

"I don't really love badgers because they are furry and good-looking. It's not about that. They are appealing, there's no doubt, they are like little bears, especially when they are young. To me they are fascinating and rather mysterious because they have been in the British Isles longer than humans and they have their own social ways, not all of which is understood by us.

"I can't help but have a sort of awe for all wild creatures who have survived even the awfulness of what we have done to the world. We are the vandals in this world, there's no doubt about it."

Despite being the first wild animal to be given legal protection in Britain, in 1973, the illegal "sport" of badger baiting and digging still goes on, and this year killing badgers is set to be sanctioned by the government – which wants to authorise farmers to trap and shoot them to reduce bovine TB. May is convinced this is the Conservatives' political sop to the countryside lobby because, locked in coalition, they lack the numbers to repeal Labour's hunting ban. "It's a panacea that is being offered to farmers, look we are doing something, we are on your side, we're going out and killing things," he says.

Bovine TB led to the slaughter of 24,899 cattle in England last year, costing £63m. Farmers insist the disease is a genuine crisis, and argue it has increased with a burgeoning badger population and that disease hotspots correspond to high badger populations, particularly in the West Country. May insists that it is still unproven that badgers pass TB to cattle (it is proven that cattle transmit it to badgers) and unproven that a cull would help.

He quotes the conclusion of a 10-year culling trial in which 11,000 badgers were killed: culling cannot meaningfully contribute to the control of TB.

After travelling to Cardiff to unsuccessfully plead with the Welsh assembly to reverse its own separate decision to cull badgers in Pembrokeshire, May admits he "got into a lot of trouble" for suggesting farmers should abandon cattle farming in bovine TB hotspots. "People called it a ridiculous idea but it came from a conversation I had with a farmer who had already switched from cows to sheep." You would not knowingly bring up children in an area where there was radiation pollution, he says. "There was a nasty little piece" in the farming press that said telling farmers they should not farm cattle is "like telling Brian May he can't play guitar and they said some people would welcome that. Ha ha, lovely. But there was a time when Queen was very uncool in Britain and what we did was play elsewhere. I actually took my family and my little boy went to school in LA partly because of that, so it's not such a ridiculous suggestion."

"People talk about a science-based cull. I'm afraid to say people don't know what science is," he sighs. He believes bovine TB can be solved with vaccination – of badgers and cows. But the vaccination of cows is tricky: EU legislation forbids the export of vaccinated cows because it is difficult to distinguish between a vaccinated cow and one carrying bovine TB. In contrast, the vaccination of badgers is "eminently doable", argues May. "All the research has been done. This business of 'it's not proven' is really rubbish." A badger vaccine is currently being trialled in Gloucestershire although the coalition cancelled five other trials put in place by Labour's "exemplary" environment secretary Hilary Benn. The National Trust recently announced plans to vaccinate badgers on its land in Devon.

May once said he stopped voting Conservative because of the party's attitude towards wild animals. Now he says he is "very close" to the group Conservatives Against Fox Hunting, "who have the courage to stand up against the party line" but relates to the Green party more than any other. Did he vote Green at the last election? "You can't ask me how I voted! It's a secret vote. The truth is, I'm really not political. I just care about the animals. That's my party. I'm a party that cares about changing attitudes to animals."

May has all the accoutrements of a multi-millionaire rock star – assistants running around, a stable full of gold discs and, incongruously, huge pink stems of rhubarb freshly picked from his organic garden – but says he scarcely gets time to enjoy the good things in his life. He has suffered depression, particularly after the death of his father and Freddie Mercury, and seems prone to pessimism, even bemoaning his follicular majesty when he has his photograph taken with his foxes. "It's disappearing now," he murmurs despondently, mussing his luxuriant curls. "The hair has seen better days."

Long nights awake debating with badger lovers and haters on the web can get "depressing", he admits, especially when he is confronted with videos of animal cruelty. "It's a thing you have to fight, being depressed about it. It's very uncomfortable to see into the minds of people who are so full of violence. I find it very upsetting. It's changed my life.Some mornings, I find it hard to deal with." He pauses. "I get over it," he shrugs and laughs at himself for the first time.