The 52-year-old singer, who rode a huge wave of popularity with a string of hit albums in the mid-70's and has recently enjoyed a resurgence with a number of soundtrack successes, first became aware that something was wrong after becoming dizzy while playing tennis at his home in Nice, France. John later became ill while flying to London to perform at the wedding of Victoria ''Posh Spice'' Adams and David Beckham.

Initial symptoms led John to believe there was nothing to worry about, though doctors continued to perform tests, later diagnosing the singer's need for surgery. John told the BBC on Saturday ''Thank God for the British medical system because I went through every test known to man to find out exactly what was wrong. Initially I thought I had sunstroke.

John has returned to his home in France for a two-month convalescence. Concerts scheduled to take place this week in Kent and West Yorkshire have been rescheduled to late August and early September.