Towards the end of the evening, the group’s two female members Agnetha Faltskog and Anni-Frid Lyngstad took to the stage to perform the 1980 hit Me and I.
To the delight of the small audience of friends and Swedish celebrities, Mr Ulvaeus and Mr Andersson then unexpectedly joined them on stage towards the end of the performance, reuniting the group in song for the first time since they split at the peak of their fame in 1982.
“It’s been a great night,” Mr Andersson told ‘Expressen’ a Swedish tabloid evening Newspaper.
“It was absolutely amazing. A lot of emotions,” agreed singer Anni-Frid Lyngstad, who lives in Switzerland with her English boyfriend Henry Smith, heir to the WHSmith stationery fortune.
“We’ve made this journey throughout our history. Benny and Bjorn in particular. It’s been very nostalgic.”
The evening, held in the 19th century hotel’s ballroom, brought together the key people from the two men’s often intertwined careers.
It was hosted by the radio DJ Claes af Geijerstam, whose band Ola & the Janglers beat ABBA to represent Sweden in the Eurovision Song Contest in 1973, a year before they won Sweden’s first victory with Waterloo.
The group’s four members have always adamantly refused to reform, despite being offered a rumoured $1 billion (£690 million) to do so at the peak of the wave of ABBA fever, which followed the success of the musical Mamma Mia!
In January, Mr Andersson rejected suggestions that the band would perform in public once again.
After a rare joint appearance of the four members for the opening of Mr Ulvaeus’ new Mamma Mia-themed Greek taverna in Stockholm, Mr Andersson was asked about ABBA reunion. “I don’t think so,” “We took a break in ‘82, and it was meant to be a break,” he said. “It’s still a break and will remain so. You’ll never see us on stage again.”
However, the group has been inching towards Sunday’s joint performance, with the joint appearance in January following a public outing for three of the four members in 2013 to mark the launch of Stockholm’s ABBA Museum.
According to Expressen, the party on Sunday began with a video featuring the highlights of the careers of the two men, followed by band’s members and their friends telling stories of their times together.
Janne Schaffer, a session guitarist for the band, told Expressen: “We witnessed a piece of music history tonight.”
The rest of the world, however, reacted with glee at the potential of a future ABBA reunion. BBC Radio 2 celebrated on Monday morning by playing a medley of hits – which made some listeners late for work.
