McCartney expected to say 'Love me do' today

The former Beatle seems poised to marry his American fiancée on John Lennon's birthday and at the scene of his first wedding

Everything is ready. The marquee has been erected. The yellow roses, sound equipment and bags of ice have been delivered. The paparazzi and Beatles fans have gathered. For today is expected to be Sir Paul McCartney's third big day – the day he marries American industrial heiress and multimillionaire Nancy Shevell, 51, in a ceremony at Old Marylebone Town Hall.

Only 30 guests, including McCartney's five children, are thought to be going to witness the wedding, which will be followed by a party in the back garden of the Beatle's St John's Wood home. The musician's brother, Mike McGear, is tipped to be best man, while his seven-year-old daughter Beatrice, by his second wife, Heather Mills, will be flower girl.

Both the date, on which John Lennon, assassinated in December 1980, would have turned 71, and the venue, where McCartney married his first wife, Linda, in 1969, are highly symbolic. McCartney, 69, posted the legally required 16 days' notice of the forthcoming marriage in September, describing himself as a "business executive" and Ms Shevell as an "executive". The couple were then eligible to marry within the year. But speculation has been mounting all week that today has been earmarked for the ceremony, with preparations evident at McCartney's home. His fashion designer daughter Stella McCartney is said to be helping choose the vegetarian meal to be served.

Ms Shevell, whose father heads a £250m haulage firm, and McCartney are believed to have been granted special dispensation to marry today, in a ceremony conducted by Superintendent Registrar Alison Cathcart. Weddings at Marylebone usually take place from Monday to Saturday only.

McCartney proposed to Ms Shevell in the spring, with a vintage 1925 Cartier ring; the couple began dating four years ago in the Hamptons, Long Island. Their relationship started after Ms Shevell had separated from her lawyer husband of two decades, Bruce Blakeman. She has a son, Arlen, 19, from that marriage.

Ms Shevell sits on the board of New York's Metropolitan Transportation Authority. After the engagement, which she described as a "total surprise", she said: "I'd love to live here [New York] but it's probably England. I still have a job here so I'll commute once a month."

The heiress's cousin, the broadcaster Barbara Walters, is believed to be in the UK for the ceremony, after tweeting: "Going away for a big weekend. Will tell you more when I get back."

Linda McCartney died from breast cancer in 1998, aged 56. On the couple's wedding day, the bride was four months pregnant with the couple's eldest child together, Mary. They arrived at the ceremony with Linda's daughter Heather, whom McCartney adopted. They had two more children, Stella and James.

At McCartney's 2002 marriage to the charity campaigner Heather Mills, more than 300 guests attended the ceremony on the Castle Leslie estate in County Monaghan. The wedding reportedly cost £1.5m. Ms Mills received a £24.3m settlement after an acrimonious split in 2008.

While McCartney's agent has declined to confirm that the wedding will take place today, it seems unlikely that fans, who started gathering in St John's Wood yesterday, will be disappointed.