Even as he received red carpet treatment in Beijing last month, lauded by China as an "old friend" and "renowned leader", Robert Mugabe was in danger of being upstaged by a colourful, charismatic presence at his side. His first lady, Grace Mugabe, sporting a series of vivid outfits during the official visit to China, was once a lowly member of the presidential typing pool. Then she caught Mugabe's eye. Now the woman better known to headline writers as "DisGrace" or "First Shopper" is making a surprise entrance on to the political stage and, it is speculated, might be central to her autocratic husband's plan to build a dynasty.

The 49-year-old was recently nominated as leader of the ruling Zanu-PF's women's league, as well as having a place on its central committee. She insists she is ready. "The time has come to show people what I am made of," she told a crowd in Mazowe, the Zimbabwe Standard reported. "People should learn to wait for their time… I had never dreamed of entering politics, but you have approached me and I am ready to go."

With her election virtually assured at a party congress in December, she will take a seat on Zanu-PF's politburo and, as Mugabe's wife, be untouchable. Tendai Biti, the former finance minister, thinks she will go on to lead Zanu-PF. After years of speculation over who will succeed Mugabe, some believe the next leader has been hiding in plain sight.

The rise and rise of Grace may reflect her husband's confidence that, at 90, he is more powerful than ever. Africa's oldest leader looks certain of his party's nomination for the 2018 election at a time when the opposition Movement for Democratic Change (MDC) is in disarray. He was recently elected chairman of a regional bloc of 15 southern African nations and looks set to become chairman of the African Union, while western powers have lifted most of the sanctions against Zimbabwe. It appears he is able to do as he pleases.

"I heard that someone asked my nephew to tell Mrs Mugabe to leave Harare," Mugabe reportedly told hundreds of cheering young people bussed to Harare airport to welcome him back from China. "Where do you want her to go? We don't want such arrogance."

Grace's ascent has also thrown a grenade into the bitter succession battle within Zanu-PF, which Mugabe has divided and ruled for decades. But even he, it is supposed, cannot live for ever. According to one theory, his wife is a weapon to derail the presidential ambitions of another woman, Vice-President Joice Mujuru, who is locked in a power struggle with justice minister Emmerson Mnangagwa.

As the infighting worsened, Simon Khaya Moyo, Zanu-PF's chairman, issued an edict: "I must warn the party leadership, at every level, not to use the media to abuse the first lady's name for whatever reason. The first family must be respected at all times."

Born in South Africa to Zimbabwean parents, Grace was a 20-year-old divorcee when she got a secretarial job in the president's office. Mugabe's first wife, Sally, was alive (she died of kidney failure in 1992) when he began their affair.

In an interview on South African television last year, he offered an oddly worded defence: "After Sally was gone, it was necessary for me to look for someone and, even as Sally was still going through her last few days, although it might have appeared to some as cruel, I said to myself, 'Well, it's not just myself needing children.' My mother has all the time said: 'Ah, am I going to die without seeing grandchildren?' So I decided to make love to her. She happened to be one of the nearest and she was a divorcee herself, and so it was. We got our first child when my mother was still alive."

The couple married in 1996 in a Catholic mass dubbed the "wedding of the century". Mugabe was 73 when she gave birth to their third child. Grace has since gone into business‚ launching a dairy range‚ and set up a children's school, but has shown little appetite for diplomacy.

In 2009 she punched a British photographer outside a hotel in Hong Kong, an incident she recalled laughingly on TV. Speaking to Dali Tambo, founder of Artists against Apartheid and son of South African anti-apartheid leader Oliver Tambo, she also denied having an affair with the governor of the central bank – saying of Mugabe, "I know he trusts me" – and rejected claims that she made big spending expeditions to Harrods in London. "It's one place that I've never bought an outfit," she said. "The only thing I've bought in Harrods? Almond nuts. Because I eat lots of almond nuts. I want to eat healthily."

Her sudden appearance on the political stage has taken many by surprise. Vince Musewe, a Zimbabwean political commentator, described it as a classic Mugabe tactic to conceal the country's deepening economic woes. "I really think this is a sideshow," he said. "The master politician has reframed the problem and taken attention away from his failures. We've got a 90-year-old with no energy and no idea of the way forward, and who's failed to groom a successor.

"To me, Grace is just a pawn. There's no way she's going to be anything after Mugabe dies. She's not educated; she does not understand the complexities of taking Zimbabwe forward; she does not even know women's issues. She's part of the elite and there's absolutely no way the security guys can accept her being president. She doesn't have the gravitas to lead Zimbabwe."

Musewe said he was aware of rumours that Mugabe, who has been in power for 34 years, wants to promote his wife to be a vice-president early next year. "That means for the next few years no one can challenge her. But the moment he drops, she's finished."

This view was echoed by Piers Pigou, project director for southern Africa at the International Crisis Group. "I just cannot see Grace Mugabe providing any kind of serious credibility for the leadership of Zanu-PF," he said. "It would make Zanu-PF even more of a laughing stock than it is in some quarters already. It would be a reflection more of desperation than anything else."

Political opponents are equally sceptical of the notion of a President Mugabe Mark II. Nelson Chamisa, organising secretary of the MDC, said: "If I look at the chances of her becoming president, I would find it easier to impregnate a man or change Europe to Africa, or Africa to Europe."

But Grace does have one fan: Dali Tambo. He said: "A businesswoman and a dairy farmer, I found Grace Mugabe to be a woman of considerable beauty who speaks frankly and openly with self-confidence and constant humour. She is a young first lady with an easy laugh and a busy manner."

Asked if she could be president, Tambo paused, then replied: "Why not? She has all the qualities. But politics is a complex thing, and I don't think it's for me to comment. It's for the people of Zimbabwe to decide."