The European Council's outgoing President has remarked that the European Union "will survive" if Britain decided to leave the bloc, but "will be dead" if France opted out of the union.
Herman Rompuy added that Britain would be worse off without the EU as taxpayers would be left with a £34bn (€43bn, $54bn) bill, for up to six years after opting out of the union - dubbed a "Brexit".
"Without the United Kingdom, Europe would be wounded, even amputated – therefore everything should be done to avoid it," he said.
"But it will survive. Without France, Europe — the European idea – would be dead."
Regarding Britain, Rompuy added that "everything should be done to avoid it [an EU exit]."
Prime Minister David Cameron promised voters that the Conservative party would hold an in/out referendum in 2017, if Britons voted for the party in the May 2015 general election.
Britain's relationship with the EU has become fraught over the last few years, over immigration issues and Brussels' power to push through new laws that impact the UK directly.
The Tories, as well as Labour and Liberal Democrats, are under increased pressure to curb immigration amid growing support for political parties, such as Ukip, that are in favour of stricter border control and exiting the union.
Cameron pledged to have "one last go" at curbing EU immigration laws to stop the flow of migrants into the UK.
"We need further action to make sure we have more effective control of migration," he added.
Anti-EU group Ukip's leader Nigel Farage said: "It is impossible to change the free movement of peoples within Europe without a fundamental treaty change with 27 other European countries.
"Nobody wants it, nobody is interested, and the Prime Minister knows it's not possible."
German Chancellor Angela Merkel's office wants Britain to remain part of the EU, but will not negotiate on immigration.
Merkel's spokesperson, Steffen Seibert, said Germany still wants the UK to remain as an "active and engaged" member of the EU, but warned it would not subscribe to strict immigration measures Cameron is planning to implement.