By Michael Holden

LONDON (Reuters) - The United Kingdom could break up, with Scotland going its own way, if Britons vote to leave the European Union, a longstanding EU critic and former Conservative leader warned on Wednesday, saying as a result he was likely to vote to stay in.

The comments from William Hague, who once campaigned vigorously to keep Britain out of the euro zone, followed hard on the heels of a similar warning against "Brexit" from the EU by former Conservative prime minister John Major.

In an article in the Daily Telegraph, something of a bible for the generally Eurosceptic Conservative Party, Hague said voting to quit the EU in a referendum likely to be held next year could help the Scottish nationalists' attain independence.

He said he would back remaining in the bloc despite his own serious misgivings about the EU when the referendum is held.

Hague, a former foreign secretary, also said Britain leaving the EU would weaken the bloc itself - something he did not want to see.

"We will have to ask, disliking so many aspects of it (the EU) as we do, whether we really want to weaken it, and at the same time increase the chances, if the UK left the EU, of Scotland leaving the UK," Hague wrote.

Despite the EU's many failings, he said, it provided stability for fragile democracies in central Europe. It would also not be in Britain's interests for the bloc to fall apart with such volatility in the Middle East and the world's economies.

Prime Minister David Cameron met fellow European leaders in Brussels last week to try to drum up support for a reform of Britain's relationship with the EU, prompting a number of senior Conservatives to publicly express their opinion on an EU exit, including former defense secretary Liam Fox saying on Sunday it was time to "end the pretence" that Europe would change to accommodate Britain.

Charles Grant, director of the Center for European Reform think-tank, said Hague was well respected in the party and noted his comments, coming days after Major, could be designed to move the debate beyond Cameron's tricky drive for EU reform.

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"There will be a transition from saying 'I want to reform the EU so it will work better' to saying 'the EU is a good thing in itself'," he told Reuters. "It's no coincidence that Major and Hague have come out in this way."

Hague said Scottish nationalists, who strongly support staying in the EU, would use a vote for Brexit as grounds to hold a second referendum on independence, rejected by 55-45 percent of voters in Scotland in September last year.

Scottish First Minister Nicola Sturgeon has said a second referendum would be almost inevitable in such circumstances and Hague said the outcome would be too close to call.

Nearly half of Britons are leaning towards voting to leave the 28-nation bloc a survey last week found, the latest poll to suggest that anti-EU sentiment is rising and the result will be on a knife edge.

(Additional reporting by Kate Holton; Editing by Jeremy Gaunt)