This was supposed to be one of the most glittering events at the Diamond Jubilee. Instead it has cast a rather unfortunate shadow of controversy over the celebrations. It's a gathering of 62 members of various royal families from this country and from across the world.
The focus of course is on the King of Bahrain who has been condemned by his opponents for abuses of human rights. And he was among the royal dignatories who arrived this morning and was met by the Queen and her son Prince Andrew.
And he's not the only controversial monarch who is here. Another one is King Mswati of Swaziland, a man who is accused of living a 'lavish lifestyle' while people face starvation in his own country. So it has injected a very unwelcome air of controversy in the celebrations.