Murdoch's Sun backs Cameron in England, nationalists in Scotland

(Recasts, changes date, adds Scottish edition backing SNP)

LONDON, April 30 (Reuters) - Britain's best-selling newspaper, the Sun, urged voters in England on Thursday to back Prime Minister David Cameron's Conservatives in next week's election to keep out the Scottish nationalist SNP, at the same time as urging Scottish voters to back the SNP.

The endorsements highlighted the complexity of what is expected to be one of the closest parliamentary elections since the 1970s. Polls suggest no single party is on track to win outright, and smaller parties such as the separatist SNP are likely to hold the balance of power.

Despite declining circulations, newspapers still wield political influence in Britain, and the Sun, with an average circulation of just under 2 million a day, claims to have determined election outcomes in the past.

In 1992, its front page famously asserted it had won the contest for the Conservatives. Rupert Murdoch, whose News Corp ultimately owns the paper, takes an active interest in British politics, regularly tweeting his opinions.

Although Cameron's main national rivals are the opposition Labour Party, polls suggest the pro-independence Scottish National Party could sweep all 59 parliamentary seats in Scotland, stripping Labour of the 40 seats it has left from the 2010 election, but the Conservatives of only one.

The SNP has said it would be prepared to prop up a minority Labour government in London, but not a Conservative one.

Even though Labour has so far rejected its overtures, the Sun's English front page still painted a picture of a "nightmarish Labour government, propped up by the saboteurs of the SNP", while the Scottish Sun said the SNP "will fight harder for Scotland's interests at Westminster, offering a new hope for our country".

The Sun's London editions also cited the economy and Cameron's promise of a referendum on European Union membership as reasons for backing the Conservatives.

Under the headline "It's a Tory" (Conservative), the paper ran a picture of Cameron swaddled as an infant, a play on the nation's wait for Prince William's wife, Kate, to give birth to a second child.

"Today, after a gruelling five-year wait and an appalling Labour, The Sun is proud to deliver our choice for the election," the newspaper said. "It's the Tories."

The Scottish Sun pictured Sturgeon on its front page as Princess Leia from the Hollywood film Star Wars.

Since Scots rejected independence in a referendum last year, support for the SNP has surged. On Sunday, Murdoch said Scotland was likely to win self-rule in the next few years. (Reporting by Guy Faulconbridge, William James and Andrew Osborn; Editing by Kevin Liffey)