It would be difficult to find two less likely converts to the state of wedded bliss than the actress Helen Mirren and the comedian Jo Brand. Mirren, despite her legions of middle-aged admirers, always said she was not the marrying type. Brand has built a career around venomous sideswipes at the male of the species. Yet both, to general astonishment, took the plunge over the festive season.

Mirren wed her partner, the film director Taylor Hackman, in a remote church in the Scottish Highlands on New Year's Eve. Brand reportedly told friends via a line in their Christmas cards that she had just tied the knot with her boyfriend, Bernie Bourke, a psychiatric nurse.

Two very different women have become unexpected standard-bearers for the institution of marriage, bucking the trend of long-term decline in the number of couples prepared to tie the knot. What made Mirren recant, at the age of 51, after 12 years of cohabitation with Hackman? What was it that prompted Brand to hoist the white flag and agree to share her bed with the enemy?

None of the conventional reasons apply. Neither of these successful and independent women needed a mate to provide a home, a raison d'etre or financial security. Living in sin lost its stigma long ago. Nor are there children on the scene, the advent of which often acts as an incentive for couples in long-term relationships to renounce their ideological resistance to marriage. Mirren is well past child-bearing age and Brand, as far as one knows, is not pregnant.

Mirren's case is particularly interesting because it reflects a growing perception that a small social revolution is stirring. Despite the gloomy statistics, despite the number of times that marriage's obituary has been written, there is anecdotal evidence that the wedded state is creeping back into fashion. Couples, particularly older couples who may have been together for more than a decade, are confounding their friends by slipping away to the register office and plighting their troth.

According to Oliver James, the clinical psychologist, hostility towards marriage is rooted in the parental example. If you abhor the values espoused by your parents, you reject the institution that they represent. But as people mature, he says, they tend to soften and grow more tolerant, and may end up emulating the generation that they once despised.

At 39, Brand, despite her bolshy, man-hating image - one of her gags is "never trust a man with testicles" - is past the stage of youthful rebelliousness. Her own parents split up about 20 years ago. She had only been seeing Bourke for about a year but, like many offspring of broken homes, she may have craved the security conferred by a public affirmation of love and commitment. Perhaps the ticking of her biological clock had begun to deafen her. Maybe at heart she is just an old-fashioned, middle- class girl.

Mirren, who seems to grow more alluring with every year that passes, told the American magazine People only two months ago that she could never see herself wearing a wedding ring. Not married before, and with no children, she was once quoted as saying: "I think marriage is a miraculous and wonderful thing. But, like lobster, it's just not to my taste. I don't see what everyone gets so excited about." Hackford, who shares a home with her in Los Angeles, had also said that he saw no need for nuptials.

Then they went ahead and did it after all. Perhaps Mirren, voted the sexiest woman on television earlier this year by Radio Times readers, held out for so long because, aware of her physical charms, she never felt the need to grab the first good man on offer. Maybe, given the soaring divorce rate, she feared failure - or, like many happily cohabiting couples, was anxious that marriage would change things, exert new and dangerous pressures that would end up wrecking a successful relationship.

Why walk up the aisle, in the face of so many compelling reasons to avoid that perilous trip? Virginia Ironside, The Independent's agony aunt, believes that some couples opt for marriage after years of living together in order to spice up a mundane relationship. "It's saying: `let's take another risk with our lives together, let's get even closer'," she says.

The prognosis for such couples is, unfortunately, not rosy. Men and women who tie the knot after living together are far more likely to get divorced than those who dive straight into wedlock. Virginia Ironside thinks that some underestimate the impact that getting married will have on their lives. "It's a huge step," she says. "People don't take account of the enormous difference that marriage makes to a relationship. The outside world sees you in a very different light, and you're not prepared for that."

Mirren and Brand have decided to ignore the doom merchants, and are instead taking their cue from the ancient Greeks. For, as one passage in Homer's Odyssey has it: "There is nothing nobler or more admirable than when two people who see eye to eye keep house as man and wife, confounding their enemies and delighting their friends." Why did these two women decide to get married at this point in their lives? Perhaps no one ever asked them before.