The Prince of Wales and Foreign Secretary David Miliband have cancelled their journey to the funeral of Poland's President Lech Kaczynski.
The pair had been due to join leaders from across the world for Sunday's ceremony being held in the southern Polish city of Krakow.
The statesman and his wife Maria died last weekend along with 94 others in a plane accident in Russia.
A Clarence House spokesman said the Prince was "very saddened and disappointed" not to be able to attend because of the disruption caused by the Icelandic volcano.
In a statement Mr Miliband said he and the Prince "deeply regret" that it was "impossible" to make the journey.
Earlier, some 100,000 Poles filled Warsaw's biggest public square, joining together for a memorial and funeral mass for the 96 people killed in the plane crash.
The crowd in Pilsudski Square waved white-and-red Polish flags with black ribbons of mourning affixed to them. A massive white stage, with a large cross in the centre, was flanked by oversized photos of the dead, including President Lech Kaczynski.
The names of the dead were read aloud, starting with the president and his wife, Maria, while Marta, their only child, and Jaroslaw Kaczynski, the president's twin brother and former prime minister, looked on. Others at the service included former President Lech Walesa, Prime Minister Donald Tusk and acting president Bronislaw Komorowski.
"Our world went crashing down for the second time at the same place," Mr Komorowski said of the crash near Russia's Katyn forest, site of a World War II massacre of Polish officers.
Mr Tusk called the crash a calamitous event that was "the greatest tragedy in Poland since the war". The crash claimed the lives of a swathe of Poland's elite, including numerous politicians, the central bank governor, the commanders of the country's armed forces and the head of its Olympic committee, among others.
