The 48th annual London Film Festival opened last week with a gala screening of Vera Drake, the latest film from Mike Leigh, the director of Secrets and Lies and Topsy Turvy. The film won prizes for best film and actress at the Venice Film Festival, and star Imelda Staunton is being widely tipped for an Oscar nomination for her moving, nuanced performance.

Unlike most of Leigh’s work, the film is explicitly issue-led. It tells the story of Vera (Staunton), a backstreet abortionist in fifties London, who is prosecuted after a woman she helps is taken to hospital. Performing an abortion was a criminal offence until the 1967 Abortion Act, carrying a minimum sentence of eighteen months.

Vera Drake is likely to cause controversy, but Leigh insists that it's a drama rather than a polemic. “First of all, my job is to present you with the moral dilemma which you have to confront," he says. "I don’t think these things are black and white.”

Beginning slowly, the film follows Vera in her daily routine. She rushes from one cleaning job to another, working with the energy of someone who believes that work has a moral value of its own. She stops to make tea and plump cushions for some of her elderly neighbours, and rushes home in time to cook for her family.

This consists of husband Stan (Philip Davis) and two grown-up children, Sid (Daniel Mays) and Ethel (Alex Kelly), and we learn their stories while we wait for Vera’s to begin. Stan works as a mechanic for his brother, and the financial implications of being an employee rather than partner are clearly a source of embarrassment for both men.

Sid is a genial tailor’s assistant, who trades nylons for cigarettes at the pub. Ethel is meek and mousy, and Vera finds time to set her up with Reg (Eddie Marsan), a good-natured neighbour who seems even shyer.

With the languid pace that Leigh fans will be used to, the film lays out these and other stories without hinting at the revelations to come. Half an hour in, we learn Vera’s secret: one of the ways in which she helps people out involves  performing abortions for working-class women free of charge. These are organised by Lily (Ruth Sheen), a hard-faced wheeler-dealer, who neglects to tell Vera she charges the girls two pounds for the service.

Alongside this, we see Susan (Sally Hawkins), the daughter of one of Vera’s wealthy cleaning clients, who finds herself pregnant after a sickening date rape. With the help of her aunt, she pays a hundred pounds for a legal abortion through a psychiatrist. The film’s suggestion is clear: criminalising abortion only prevents the poor from getting treated.

Vera Drake marks the latest progression in Leigh’s career, as it’s visually far more impressive than any of his previous work, which too often betrays his years in television. The period detail is meticulous, and the film uses its locations and details to powerful effect; in one scene in particular the cramped nature of Vera’s house is central to the tension of the scene.

A more traditional Leigh strength is the quality of the acting, which comes from months of improvisation and rehearsal. Imelda Staunton gives a career-making performance, presenting Vera as a meek, anxious woman, who doesn’t see the value of thinking too much. Staunton is likely to win the plaudits, and perhaps the Oscar, but the supporting cast members are equally accomplished. As a result, every scene in the film carries the simple pleasure of watching first-class actors at work.

Leigh is also renowned for his dialogue, which is consistently authentic and loaded with comic repetition and misunderstanding. Characters rarely speak directly of the thing that’s on their minds, so the scenes are charged with subtext, and this gives the actors freedom to reveal character in more subtle and expressive ways.

Leigh's talent for this type of dialogue is most apparent in the charming scene when Reg plucks up the courage to propose to Ethel. On the page, this scene might make Reg look a brute, as the proposal appears distinctly unromantic. However, on film, the contrast between the characters’ inability to communicate, and their obvious tender feelings for each other, give the scene a genuine emotional power.

In every respect, then, this is Leigh’s most impressive film to date. However, despite its obvious strengths, Vera Drake  lacks the emotional impact of Leigh’s best work, like Life Is Sweet and Secrets and Lies. The problem lies in the story, which is relentlessly gloomy and downbeat and lacks the moments of breakdown and reconciliation that were so crucial to previous successes.

Those films were entirely character-driven, but in Vera Drake  the presence of an "issue" is too apparent, and the film’s message comes across as direct and heavy-handed. As a result, the film refuses any opportunities to allow Vera a moment of triumph or struggle. Instead, she becomes a martyr, and as she becomes passive we start to lose sympathy for her. Leigh may not think so, but Vera Drake is a polemic, and suffers accordingly.

Nonetheless, this is an important and moving film, which seems certain for recognition in next year’s Oscars. It may also be the most heartfelt work yet from one of our finest filmmakers. The film ends with a dedication: “In loving memory of my parents: a doctor and a midwife.”

(Released by Fine Line Features and rated "R" for depiction of strong thematic material.)