A spokesman indicated earlier this week Cameron would opt for a business suit at the April 29 event instead of the traditional tails associated with upper-class English weddings.
But Cameron's office said Wednesday the prime minister now plans to wear a more formal suit. His decision is expected to lead other politicians to shed business wear for morning suits.
The wedding invitation gives men a choice of suits, military uniforms or traditional formal wear.
Cameron has been reluctant to wear outfits that remind voters of his elite education at Eton College and Oxford University.