EU president Donald Tusk warned Sunday that British Prime Minister David Cameron's plans to amend the bloc's treaties to secure a new deal for Britain in Europe were virtually "mission impossible".
"My intuition is that treaty change is close to mission impossible today because it's not only about rationality, about good argument," Tusk said in an interview with The Guardian newspaper published online.
"We need unanimity between 28 member states, in the European parliament, in 28 national parliaments in the process of ratification. To say that it is a Pandora's Box is too little."
However, the former Polish leader said: "I am ready to help Cameron... I have no doubt that we have to help in a limited and rational framework.
"We have to help David Cameron because he is obviously pro-European. I am sceptical when it comes to changing the treaty."
UKIP leader Nigel Farage has reportedly promised to support Cameron's party in the event that it does not win a parliamentary majority in May, on the condition that it brings the EU referendum forward to 2015.