Dr Who saves the Earth (and joins the protests against the war in Iraq)

Tony Blair may prefer to ride out Christmas Day's Doctor Who special by sheltering behind the No 10 sofa. For one of the highlights of the BBC's festive schedule will contain a pointed anti-war message and raise the suggestion that the Prime Minister is a poodle of the US President.

Russell T Davies, the chief scriptwriter, said the hour-long show - the first to star David Tennant as the new Doctor - "absolutely'' included an anti-war message "because that's what I think". "It's Christmas Day. Have you read the Bible? It's a day of peace," he said.

In the prime-time special, the Doctor is called on to help repel an invasion by a particularly ugly race, the Sycorax. In a somewhat nostalgic interpretation of modern power politics, the newly elected Prime Minister Harriet Jones, played by Penelope Wilton, is in charge of handling the threat. When her assistant informs her the US President is on the telephone and wants to take control of the situation, she replies in no uncertain terms: "Use these exact words - 'He is not my boss and he is certainly not turning this into a war'."

The Prime Minister's pacifist instincts are overridden when, in an echo of Margaret Thatcher's decision to attack the General Belgrano during the Falklands conflict in 1982, she orders the destruction of a retreating alien spaceship. The Doctor, who opens the drama regenerating in bed, while his assistant Rose Tyler faces an evil trio of masked Santas and a killer Christmas tree, is disapproving and ensures that she is swiftly declared "unfit for duty.''

According to Mr Davies: "She [Prime Minister Jones] does that very easy speech about not listening to the American president, but at the end she's out of her depth and she does the wrong thing.''

As well as the implicit reference to Mr Blair's support for President George Bush over the invasion of Iraq there is a plot line involving a new secret missile defence system called Torchwood.

It is all a far cry from 40 years ago when the first Doctor Who episode to be broadcast on Christmas Day, called "The Feast of Steven" and starring William Hartnell as the Doctor, steered clear of political controversy.

But while the sight of the Sycorax leader - an unpleasant mass of muscle and bone - could unsettle young children as well as Mr Blair, Mr Davies said the BBC had been "very careful'' in deciding how far to take the horror element. Peter Fincham, the BBC1 controller, said the show had brought back "family viewing'' to the channel.

"Doctor Who has rediscovered something we had lost on BBC1 which is family viewing. When Doctor Who started suddenly it was there again," he said.

"I would compare Doctor Who with films like Toy Story or Shrek which have enormous appeal to children but manage to look at adults eye to eye."

The BBC1 chief praised the new Doctor. "David Tennant brings wit, heart and intelligence to the role of the Doctor,'' he said. He also paid tribute to the actor Christopher Eccleston, described as "an extraordinary Doctor too ... he completely launched Doctor Who for the 21st century.''

Indicating that he wants to stay in the role for the foreseeable future, Tennant said: "I fully intend to be here next year.''

The new Doctor's role is more light-hearted than the part played by Eccleston. He dresses in a shabby brown overcoat, pinstripe suit and plimsolls bought in a second-hand shop. In one of his first lines after he has regenerated, changing his appearance, he jokes that he wanted to come back with ginger hair.

The second series of Doctor Who, which is due to be shown next year, features a race of cat women, the return of the robot dog K-9, Queen Victoria and a kiss on the lips between the Doctor and Rose.