Amy Winehouse (left) was not scheduled to perform at V
Amy Winehouse performed on stage at the V Festival in Essex on Saturday, a year after she last sang in public in the UK at the same festival.
The singer, who recently returned from several months in St Lucia, joined The Specials for a medley of their tracks You're Wondering Now and Ghost Town.
The crowd cheered as the singer came on stage, introduced by the band as "our good friend Amy".
The Specials will appear at V's Staffordshire site on Sunday.
A spokesman for Winehouse said: "While living in St Lucia, Amy missed the band's reunion tour and decided to head to V to catch her favourite artists.
"Meeting them backstage, they invited her to guest for them and made her dream come true," he added.
Winehouse was seen on stage earlier in the day alongside Pete Doherty but she did not sing.
Her successful duet with The Specials was in stark contrast to this time last year, when Winehouse was booed by the V crowd in Staffordshire, although her performance in Essex on the Sunday was well received.
In May this year, Winehouse cancelled her appearance at a concert to celebrate the 50th anniversary of the Island Records music label.
Earlier that month, the pop star was forced to abandon a concert in St Lucia because of poor weather conditions.
However, members of the audience criticised her performance after she appeared to forget the words to a song halfway through and told the audience she was "bored".