The news led to a wave of online congratulations from Saudi Arabia and across the Islamic world, with many Muslims saluting Mr Collis as “Haji Simon” using the title reserved for those who make the pilgrimage to Mecca.
Mr Collis converted in 2011 shortly before marrying his wife, who is Syrian. While his conversion was known to some fellow diplomats it was not public knowledge in Riyadh.
The Foreign Office declined to comment, saying Mr Collis’s religion was a personal matter.
While Mr Collis acknowledged many of the congratulatory messages coming in on Twitter, he declined interviews about his faith.
The pictures of him and his wife were first posted by Fawziah Al-Bakr, a Saudi women’s rights activist. “The first British ambassador in Saudi Arabia is performing the Hajj with his wife Mrs Huda after he converted to Islam. Thanks be to God,” she wrote.
He is believed to be the first British ambassador to carry out the Hajj pilgrimage although other ambassadors are thought to have converted to Islam in the past.