The King expressed his sadness when he met survivors of the Manchester synagogue attack during a visit to the congregation to deliver his sympathy in person.
Charles was introduced to the “heroes” by Rabbi Daniel Walker – a group of senior figures from the Heaton Park Hebrew Congregation Synagogue who helped stop Jihad Al-Shamie’s terror attack on the on the holiest day in the Jewish calendar.
“I can’t tell you how sad I am,” the King told the group, which included the synagogue’s chairman Alan Levy, who helped to barricade the door, and its president Hilary Foxler.
And later when he met other members of the congregation, the King commented “terrible thing to come out of the blue”.
When the attack began, Adrian Daulby, 53, leapt from his seat in the synagogue to block the doors of the Jewish holy place as knife-wielding Al-Shamie, 35, tried to get in to attack worshippers.
Melvin Cravitz, 66, was killed along with Mr Daulby, as the attack unfolded on Yom Kippur, the most holy day in the Jewish calendar, with many worshippers inside the synagogue in Crumpsall, north Manchester.
Mr Daulby is believed to have been inadvertently shot dead by armed police as they scrambled to the scene to stop Al-Shamie, who had driven his car at worshippers outside, attacked others with a knife and tried to storm the synagogue, wearing a fake suicide belt.
Yoni Finlay, 39, is also believed to have been injured by a police bullet and was discharged from hospital last week but looked well as he met the King with his parents.
He said afterwards: “Three weeks ago I saw the worst of humanity, but since then you do absolutely see the best, and there’s a lot of good people out there.”
Charles was making his first appearance at an official engagement since Prince Andrew’s decision to relinquish his titles left the King’s brother under intense public scrutiny.
Andrew attempted to draw a line after years of controversy, following allegations he sexually abused Virgina Guiffre, by giving up his dukedom and other honours on Friday ahead of the publication of Ms Guiffre’s posthumous memoirs.
The prince vehemently denied the allegations.
