Hollywood actor Robert Downey Jr amazed children at Great Ormond Street Hospital by making a surprise visit to their cancer ward.

The Iron Man actor, 51, was in London ahead of the European premiere of Captain America: Civil War on Tuesday, but took time out to visit young patients and their families.

Ethan Miller, a seven-year-old fan who suffers from a rare blood disease as well as type 1 diabetes, was "over the moon" to meet the American star.

Robert Downey Jr has played Tony Stark (aka Iron Man) since 2008, but it's not the first time the actor has used his fame to surprise young fans.

Last year the actor visited Alex Pring - a then seven-year-old, who was born with a partially developed right arm. Showing up as Tony Stark, he presented Alex with a bionic Iron Man arm and called him "the most dapper 7-year-old [he's] ever met."

Great Ormond Street Hospital is one of the world's leading children's units, treating young patients with rare, complex and life-threatening conditions.