Sources close to Elton tell TMZ the singer is set to perform during Saturday's event ... it's still unclear whether he'll get behind his piano at the church ceremony or at the reception later in the night.

Elton's been tight with the Royal Family for years ... he was an extremely close friend of Princess Diana. The two worked together to raise money for AIDS research and awareness of the disease.

The singer was also a tremendous source of support for Prince Harry and Prince William when Diana died in 1997. Elton famously performed "Candle in the Wind" as a tribute for Diana at her funeral.

Elton had originally booked shows in Vegas for this weekend, but they were rescheduled months ago due to a "scheduling conflict" ... guess now we know the conflict.