Doctor Who star Matt Smith has a new mission: to boost the sci-fi show's popularity in the US.

Matt, 27, flies to New York this week with Karen Gillan, who plays sidekick Amy Pond, to promote the series before it begins on BBC America on April 17.

A source said: "Doctor Who has a cult following in the States thanks to David Tennant, but bosses are really hopeful that Matt and Karen can help to make the series even bigger.

"If the Doctor can work his magic, the pair will become worldwide stars."

The new Time Lord impressed more fans in his debut episode than his predecessor, Tennant, according to a BBC survey of 5,000 fans.

Matt's first show scored 86 out of 100, while Tennant's debut, in The Christmas Invasion, got 84, and Christopher Eccleston's first story scored 76.