Sources close to Cowell indicated the relationship was at an end in the wake of Clifford's conviction on eight counts of indecent assault earlier today.
A spokesman for Syco, his joint venture with Sony Music Entertainment, declined to comment on the claims.
Cowell did not appear at Clifford's trial or provide a character witness statement.
A spokesman for Clifford also declined to comment, citing his lawyers' advice not to speak. However, in an interview with PRWeek conducted earlier this month during the case he all but conceded that a conviction would threaten what he said was a 12-year relationship with the music industry boss.
"At least a couple of clients will no longer be clients because [they are assisted] by me hands-on," he told PRWeek. While he declined to comment on whether this group included Cowell, he claimed he was involved in "everything to do with Simon's personal publicity" and this was entirely handled by him rather than his agency staff.
Clifford also said his PR agency, Max Clifford Associates, was "self-contained" enough to continue without him as another four or five clients are handled by his staff with only a small contribution from him.
These include Boux Avenue, the lingerie brand owned by Dragons' Den businessman Theo Paphitis, the Miss World Organisation and Dave Fishwick, around whom Channel 4’s ‘Bank of Dave’ show is built.