Leaving the European Union could lead to the break-up of the United Kingdom while "gravely weakening" its former continental partners, former foreign secretary William Hague has warned.

Writing in The Daily Telegraph, Lord Hague said Scottish nationalists would jump at the chance to re-open the independence debate if there was a vote to leave the EU in the forthcoming referendum on Britain's continued membership - now expected next year.

At the same time, he said, the loss of one of the EU's only two "respected military powers" would leave the bloc seriously weakened at a time of political turbulence and economic volatility.

"To end up destroying the United Kingdom and gravely weakening the European Union would not be a very clever day's work. So, even as a long-standing critic of so much of that struggling organisation, I am unlikely in 2016 to vote to leave it," he wrote.

Lord Hague is the latest senior Conservative to enter the increasingly intense debate on Britain's EU future, amid warnings it could open up a new "civil war" within the party.

While he insisted that he remained a Eurosceptic - describing the EU as "remote, expensive and over-regulating" - he said it was "manifestly not in our interests" for it EU to fail.

"There is no doubt that without the United Kingdom, the EU would be weaker," he wrote.

"We will have to ask, disliking so many aspects of it 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.

"Scottish nationalists would jump at the chance to reverse the argument of last year's referendum - now it would be them saying they would stay in Europe without us.

They would have the pretext for their second referendum, and the result of it could well be too close to call."

Meanwhile Conservative Party vice-chairman Mark Field rejected calls for ministers to be given a free vote in the referendum and said that any who were not prepared to back David Cameron's re-negotiation should resign from the Government.

"I have the greatest respect for all those in my party who believe we should leave the EU, but clearly such a position is incompatible with holding a ministerial office in advance of the referendum," he told The Daily Telegraph.

"What I don't accept is individual ministers believing they should publicly have a free rein or that they can, in an off-the-record way, spend the next few months undermining the Prime Minister's package of renegotiation. That is quite wrong."