In his previous novel, South of the River, Blake Morrison cast a shrewd eye over the landscape of England in the first half of New Labour's reign, charting the lives of his characters as they negotiated the terrain of our recent history, through landmarks vivid from news headlines. From the panoramic sweep of that state-of-the-nation novel, he has zoomed inwards, focusing tightly in his new book, The Last Weekend, on two fortysomething couples over one stifling August bank holiday as the New Labour epoch hobbles towards its close.

The Last Weekend is an age-old tale of obsession, jealousy and revenge given a sharply contemporary flavour by the concerns that drive events to their climax. Ian Goade, the narrator, who tells his story retrospectively, is invited with his wife, Emily, to spend the last weekend in August with Ollie and Daisy, Ian's friends from university, in a country house in East Anglia. Ian is at first reluctant; the gulf between his own life and Ollie's serves to reinforce a multitude of insecurities concerning his status and his masculinity. But Emily persuades him that it will do them good to leave their anxieties behind for a weekend.

In perfectly controlled increments, Ian's narrative reveals the underlying fault lines that contribute to the growing tension and creeping unease that besets the weekend party. There is the history of his relationship with Ollie, a textbook case of class envy; from the beginning, Ian has both craved and resented the sense of patronage that has always marked their friendship. Beyond this lies a festering sore of jealousy over Daisy, with whom Ian had once been in love and tentatively dated before fatally introducing her to his friend. Added to this are the cracks in Ian's own marriage, caused ostensibly by his and Emily's failure to conceive, but also by underlying financial worries.

Topography and weather are employed to careful effect: the oppressive heat and subsequent storms are an obvious pathetic fallacy for the increasing claustrophobia inside the house, as resentments that have lain dormant for 20 years begin to bubble up. The catalyst is Ollie's early confession to Ian that he has terminal cancer, followed by a challenge: a sporting contest for money, to be carried out over the course of the weekend. In a show of bravado, Ian raises the bet to £10,000, money he does not have, so the wager assumes a significance beyond the longstanding rivalry between the two men.

What appears at first to be a small story of middle-class rivalry quickly turns into something more chilling. Morrison handles the elements of his novel with impeccable control. The slow-burning feeling that nothing is as it should be, present from the outset, is joined by a further discordant note as the reader becomes uncertain as to whether Ollie is telling the truth about his tumour. Is he also lying about having visited the run-down old farmhouse as a boy, on the holiday when his father drowned?

Ian is certain that Ollie reinvents the truth for his own purposes but the reader, offered only Ian's perspective, gradually forms the nasty suspicion that the narrator might also be guilty of self-delusion and misrepresentation.

It would be a disservice to such a tightly woven plot to give away any more detail, except to say that the reader quickly absorbs the knowledge that tragedy is no more avoidable than a thunderstorm after a weekend's sultry heat. The Last Weekend is the more frightening for the fact that its tragedy is rooted in such apparently ordinary motives, motives that, Morrison suggests, are both timeless and given new potency by the demands of our age.

How long one can live happily on a diet of crumbs from the rich man's table before exploding with rage and resentment is the question that animates Blake Morrison's creepy and disturbing novel. It's played out against the ostensibly benign setting of a country weekend during which two couples meet, after a gap of some years, to renew the bonds of friendship and enjoy an interlude of bucolic relaxation. Even in real life, such occasions are rarely the simple social exchanges we might wish or pretend them to be: the extending and accepting of largesse is often involved, the reinforcement of an unspoken hierarchy, thickened by the nuances of past personal history and the irrepressible concerns of the present day. But in fiction, a situation so open to complication provides the novelist with the perfect opportunity to run riot through themes of rivalry, jealousy, class tension and hidden desire; and Morrison does not shirk that opportunity.

He begins with the names of the couples: the affluent, expansive, metropolitan Moores, Ollie and Daisy, extend an invitation to an East Anglian bank holiday idyll to the Goades, already sounding as if they might welcome temporary liberation from their provincial backwater, overloaded public sector jobs and constrained finances. But from the outset, Ian Goade, our narrator, and his wife Em have reservations about the prospect: have the Moores simply been let down by some classier friends? Will they find themselves, as once happened at a glitzy party, shunted to one side with the hired help? Unable to resist the lure of the elegance and luxury they imagine will be on offer, they decide to risk it.

In a humorous and understated sideswipe at the upper middle-class staycation, the house provides little in the way of glamour or gracious living: dilapidated and damp, it is exactly the sort of place that posh Ollie adapts to with boarding-school over-enthusiasm, his beautiful wife stoically conjuring up fabulous meals in a dingy kitchen while Ian and Em attempt to hide their disappointment. Throw in the hosts' mildly delinquent teenage son, Ian's evident romantic interest in Daisy and a surprise visitor in the shape of eager-to-please Milo, and the scene seems set for a pleasant but unremarkable comedy of manners, a satire on social aspiration with a few gropes under the stairs for good measure.

But Morrison has rather more to say on the subject of friendship, and decides to say it via that most unsettling of mouthpieces, the unreliable narrator. Ian, it emerges in deliberately measured doses of self-explication, is leaving behind more than the humdrum routine of a primary school teacher. There is the matter of the tribunal he must face on his return, convened after he physically disciplined a child and further overshadowed by possible allegations of racism; the "playing on websites" that alludes darkly to a gambling habit veering out of control; and his unease and silence in the face of Em's plans to have children. And then there are the disquieting references to past lapses in control: episodes of voyeurism, his casually recounted story of hitting a prostitute who hassled him, the way in which Em says, reassuringly, "You hardly ever lose your temper these days".

What he is not able to leave in Ilkeston, however, is the problem of Ollie, the suave university friend who stole Daisy from him and has, in the process, consigned him to a life he cannot escape. When Ian is presented, courtesy of an impulsive bet, with the chance to best his rival for good and clear his debts at the same time, the "last weekend" comes to have the feel of a day of reckoning.

Morrison's plot is not, it must be said, especially sophisticated; the image of two men, at least one of whom is probably insane, slugging it out on the golf course is clearly schematic and the novel's denouement hardly impossible to predict. But he does a terrific job of keeping the reader consistently intrigued by the strange interdependency of Ian and Ollie's relationship and the extent to which Ollie is a version of Ian who has, by circumstances of birth and luck (as Ian's obsession with betting underlines), got away with it. As our knowledge of Ian's dangerous delusions increases, so too does our suspicion of Ollie: has he really, as he claims, been diagnosed with terminal cancer? What are we to make of his insistence that he has brought the party to the very spot where his father died in a freak accident? And don't we find him as easy to dislike as Ian does?

At the novel's conclusion, it is the wives who are left to pick up the pieces, and it's tempting to conclude that Morrison has little patience for the byways of male friendship that he set himself to explore. But a closer reading might suggest a more complex sympathy for two men struggling, in different ways, under the burden of masculinity and searching desperately for a way to be released from it.