“He very much hopes they can be rescheduled in due course and offers his deepest apologies to all those who had worked so hard to make the planned visit possible.”
The palace declined to give any details of the side-effects. A royal source described Thursday's setback as no more than the “most minor bump in the road that’s very much heading in the right direction” and said it was not uncommon with medical treatments.
The king's recovery was continuing in a very positive direction, he was in good form, and was continuing to work and make calls from his study, the source said.
His state visit to Italy next month with the queen was expected to go ahead as planned, and no other health update was anticipated.
Charles' initial cancer diagnosis came as a shock less than 18 months into his reign.
However, royal aides have been upbeat about his health since and while his diary is carefully managed, he has returned to a busy schedule of work.
Renowned as a workaholic who often worked until late on his official papers, Camilla and other family members have said he needed to be encouraged to slow down.
The royal source said the king's medical team were fully involved in discussions over his schedule, saying reactions to treatment could be unpredictable.
The king's illness has coincided with that of his daughter-in-law Kate, 43, wife of his elder son Prince William, who also has undergone treatment for cancer.
She said in January she was in remission and has gradually been returning to duties.
**Reuters**