Cameron's office had said he would wear a business suit. But a source close to the prime minister told The Daily Telegraph he has had a change of heart.
"Of course he's got to wear tails. He knows that," the source said. "He's the Prime Minister, it's the Royal Family, there will be foreign dignitaries present. It is only proper that he dresses for the importance of the occasion."
The prime minister has an upper-class ancestry, including descent from King William IV and his mistress, and received an upper-class education at Eton and Oxford. As a politician, however, he often tries to submerge that background.
Cameron aides told the Telegraph they put out the word on his wedding outfit without checking with him first and pointed out that he normally wears a suit for weddings. But he wore a morning coat when he was a bridegroom.
Prince William and Kate Middleton are to be married April 29 at Westminster Abbey. Guests have been advised that either morning coats or business suits will be appropriate, although, presumably, anyone in the front row will opt for the former, the newspaper said.