For more than 40 years, Steven Spielberg has been asking audiences the same question: what if we're not alone?

From the friendly alien at the heart of E.T. to the mysterious visitors in Close Encounters of the Third Kind, some of his most celebrated films have explored humanity's fascination with life beyond Earth.

Now, the Oscar-winning director is returning to that theme once again with Disclosure Day, a sci-fi thriller starring Emily Blunt, Josh O'Connor, Colman Domingo, Colin Firth and Eve Hewson.

The film imagines a world on the brink of a revelation: proof that non-human intelligence exists and has been hidden in plain sight.

At the centre of the story are Blunt's character, meteorologist Margaret Fairchild, and O'Connor's Daniel Kellner, a cybersecurity expert who uncovers evidence of a long-running cover-up and finds himself pursued as governments and powerful corporations race to contain the truth.

Speaking at the UK premiere in Leicester Square, Spielberg told the BBC he remains captivated by the question that has inspired some of his most iconic work and his view on the possibility of extraterrestrial life has evolved over the decades.

"My view has become more realistic," he said. "There's a lot of mystery and things that are undisclosed but I've become more optimistic that people are going to be able to discover things that we have not been allowed to discover."