Leaving the EU could lead to the break up of the UK, former Foreign Secretary William Hague has warned.
In the Daily Telegraph, he said he was “unlikely” to join former cabinet colleagues Liam Fox and Owen Paterson in voting to leave in the referendum which he suggested would be in 2016.
Eurosceptic Tory MP Bernard Jenkin said Lord Hague was “completely wrong” to suggest a “Brexit” could split the UK.
The prime minister has promised to hold an in/out vote before the end of 2017.
Mr Hague’s comments came as Poland’s new President, Andrzej Duda, warned in an interview with the BBC that the EU would face a “very serious crisis” if the UK were to leave.
Mr Duda said the EU had “many weaknesses” and warned a UK exit from the bloc could cause further havoc.
“The EU has been shaken time and time again with crises – whether it’s the financial crisis, or the refugees crisis,” he said.
“Let’s not pretend, a UK exit from the EU will be a very serious crisis for the EU. Of that I have no doubt.”
In Mr Hague’s article he warned that if the UK did vote to leave the EU he believed Scottish nationalists would “jump at the chance” to re-open the independence debate, and “the result of it could well be too close to call”.
He also said a UK departure would see the 28-member bloc lose one of its “respected military powers” and leave it weakened.
“To end up destroying the UK and gravely weakening the European Union would not be a very clever day’s work,” he wrote.
“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 said.
“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.”