Queen Elizabeth II and her husband, Prince Phillip, the Duke of Edinburgh, attended a ceremony in the U.K. capital's Green Park to officially open the monument in honour of the estimated 50,000 Bomber Command fighters who lost their lives in the war against Nazi Germany.
McGregor, whose brother Colin is a Royal Air Force pilot, took to Twitter.com before the unveiling to congratulate the veterans, who have spent years campaigning for a permanent national tribute to their fallen comrades.
He writes, "It's a big day for our Bomber Command Veterans today. The unveiling of the first proper memorial in London. Congratulations and thanks. From us all."
Late Bee Gees star Robin Gibb, who died last month (May12), had spearheaded a campaign to raise the $9 million (£5.6 million) needed to build the monument.