Maggi Hambling is 70 later this year, and a career that took off when she was appointed the first artist in residence at the National Gallery, in 1980, shows no signs of slackening in momentum. Hambling is still as uncompromising as ever, and as difficult to categorise. An artist of sustained originality and inventiveness who fits no pigeonhole and is part of no group, she is a resolutely independent figure (she enjoys the description ‘maverick’) who considers it her duty to keep questioning assumptions (her own as much as other people’s) and looking afresh at the world. Occasionally she succumbs to an idée fixe — for a long while she painted sunrises as she now paints the sea — but the larger thing which is her art always emerges enriched by such lengthy immersion in a single subject. For instance, since she started painting waves, she has been trying to get the energy and crash of the sea into her other work, something she feels she achieved in the most recent paintings of her friend George Melly.

Hambling is unpredictably popular: as likely to appear on the BBC as in Pop, the ultra-stylish fashion magazine. (She features in the current edition with a 30-page interview, photographs by Jürgen Teller — though she declined to pose nude for him.) She hasn’t been promoted much abroad, though her un-English interest in death would make her an obvious candidate for exhibition in the Latin countries. Hambling’s latest work, once more engaging with the subject of death, is a protest against war. She gives it to us straight — the razed battlefields, the melting features of the victims — with no fashionable irony or theoretical mediation. She is refreshingly direct and painterly, her heart very much on her paint-splashed sleeve.

She exhibits regularly, the current show being War Requiem & Aftermath in the Inigo Rooms of Somerset House, part of King’s College London (until 31 May). In these subterranean galleries (Hambling likes the labyrinthine quality of the space and judges it well suited to the work she’s showing), are assembled paintings, drawings and sculptures she’s made on the subject of war over the past 30 years. The exhibition focuses on recent paintings of battlefields and war victims, and on a series of brightly painted, stump-like bronzes collectively entitled ‘Aftermath’, imaginative works derived from the metamorphic presences which emerge like myths from old bits of tree trunk. She says: ‘Somebody said that walking into that room of sculptures was quite unnerving because it felt as if all these ancient people were staring at them. Like a great courtroom of judges — which I quite liked.’

In England, she is famous: for her forthright opinions as much as her art. She is good on radio and telly — determined, well paced, witty. She loves to perform (she inherited this from her father), but is such a professional that it takes a lot out of her. Preparing for a broadcast or writing a birthday poem for a friend is given equal attention. All this takes her away from the studio, but for an artist who works obsessively with only the occasional break (‘I get bored to death on holidays’), such distraction is probably a good thing. ‘My reality is being in my studio either painting or making sculpture or drawing,’ she says. ‘That is my reality and the rest is the rest like a charade. In the studio, trying to get to the truth of something — that is what is real for me; a lot of the rest is unreal.’ She does occasionally travel: trips to Egypt and Mexico have been important for her work, and in Berlin in 2005 she was much moved by Daniel Libeskind’s Jewish Museum, which was an inspiration behind her own War Requiem.

Unlike some artists who are regulars on the private-view circuit, seeking company after a day’s solitary confinement in the studio, Hambling deliberately restricts the number of exhibitions she sees. She will turn up to support friends (she is remarkably loyal in this) and will visit shows by artists she admires — partly to test her reactions against the work on display, and to discover whether it still moves her and, if so, how much. She always has something pertinent (and often unexpected) to say about even the most familiar. For instance, she loved the recent Rembrandt show at the National Gallery (she affectionately refers to the artist as Ronnie), but she thought the hang was the wrong way round. ‘If I’d had anything to do with it, I would have suggested that the room of self-portraits should have been the climax of the show, not at the beginning,’ she says. ‘Those are the paintings that most directly speak to us, in the way that Hamlet directly speaks to us.’

She also enjoyed the Anselm Kiefer exhibition at the Royal Academy last year. ‘I thought it was terrific — grand and serious, weighty but beautiful, a lot of it just marvellous, marvellous paint. I know when anything is any good because I start to yawn pretty damn quickly.’ It’s the yawn of nervous apprehension — the kind of yawn that overtakes one before giving a public talk or a broadcast — not a yawn of boredom or tiredness. Hambling is a great responder. ‘I respond to what happens in life, and I’ve always refused to do what anyone might tell me to do, because I answer to some thing that’s inside me. I don’t really think that I’m in charge. It goes back to the art teacher at school, Yvonne Drewry, saying, “A subject chooses you, you don’t choose a subject.” That still goes on. In the same way I object to being called an Expressionist. I’m not an “ist” of any kind. When I’m really working, the subject has completely taken over and I’m not in charge.’

Her ‘walls of water’ paintings, shown earlier this year at the National Gallery, have divided critical opinion, which she finds stimulating. The vertical plumes of paint on these large canvases are more intensely decorative than much of her work, and show in their handling her supreme love of paint as substance. There is a parallel here with the romantic abstraction of the American painter Cy Twombly (1928–2011), whose work Hambling admires. ‘I think Cy Twombly produces in his work that mysterious territory between life and death. He takes you to another place where it is the food of the human spirit, it’s mysterious and powerful, and something other. Great art is never mundane, which is probably why I can’t stand photo-realism. I really can’t see the point of it — it’s just reportage. Reporting something, rather than responding and recreating something, which is what great art does.’

Despite her admiration for certain abstract artists (Jackson Pollock is another), Hambling rejects the notion that her own work is abstract. ‘When the sea is the subject matter of my paintings — whether walls of water or waves — how can they possibly be abstract? Part of the point of oil painting is that it requires time and the engagement of the person looking at it. In the Victim paintings I was pushing the paint and pushing the image as far as I could while still just registering the human head. So if anyone says “This is an abstract”, it means they haven’t looked at it long enough. The Victims are painted at the moment of impact — having received a wound or been hit by a lump of fire or metal. It’s very much to do with the destruction of a human head. And that is all part of my spookily old-fashioned belief that oil paint can say something in a different way from film or photography, because it’s this live stuff which, if it’s handled properly, goes on being alive. I passionately believe — whether it’s late Titian, Rothko, or van Gogh — it’s as if the painting is happening in front of you.’

There is something akin to religious belief about this. More or less every Sunday throughout her childhood, Hambling was taken to church, mostly by her mother. Her father tended to join them when he was due to read the lesson. Young Margaret looked about her, at the carvings and the stained glass, absorbing their visual lessons rather than listening to the services. Now she describes herself as ‘an optimistic doubter’. But she also says: ‘It feels like you’re talking to God if you’re trying to get to that magical territory which great art inhabits, for me — this mysterious territory which is life and death at the same moment.’ When you think about it, ‘talking to God’ is a typically punchy and enlightening definition of art, but not one that many these days would propose. Hambling — individual to the last.