Rehn's intervention – which followed similar exhortations by US and German officials earlier in the week – came as popular British tabloid The Sun reported that Cameron will react during a landmark speech on Europe later this month in The Hague.
“If I were a Briton in the EU, I would prefer to be in the midfield as a playmaker, rather than sitting on the sidelines as a substitute," said Rehn – a former semi-professional footballer – said at an event hosted by the European Policy Centre think tank. "You never score goals from the bench."
Cameron under pressure to offer referendum
Cameron wants to renegotiate the UK’s relationship with the EU, amidst pressure from eurosceptic deputies within his centre-right Conservative party to hold a referendum on EU membership.
On Thursday (10 January), the chairman of a cross-party delegation from the German Bundestag's EU Affairs committee visited London to signal his country's alarm at the danger of a possible UK exit.
"Losing the single market for the UK would be an economic disaster," Gunther Krichbaum, a member of Chancellor Angela Merkel's ruling CDU party, told reporters at the German Embassy in London.
Krichbaum also argued that Britain would suffer a significant loss of global prestige if it left the EU.
"Britain leaving [the EU] would weaken the European idea, but it would weaken Britain's position in the world more," he said.
"By the end of the 21st century, Europe will account for only 4% of the global population," he added. "We have to stand together."
Krichbaum’s remarks followed a public warning on Wednesday (9 January) from Philip Gordon, the US assistant secretary of state for European affairs, that Washington wants to see a continued strong UK voice within the EU.
Some members of the German delegation in London on Thursday said that a British departure from the EU could damage relations with Washington for both London and Brussels.
Cameron reported to enlist Dutch leader’s support
Meanwhile The Sun reported on Friday that Cameron would “hit back at President Obama’s attack on his EU referendum plan”, in a landmark speech “almost certain” to be delivered in The Hague on 22 January.
Describing Gordon’s comments as “sparking fury among Conservative MPs”, the daily claimed “Dutch leader Mark Rutte will back [Cameron’s] bid to fight for powers and money to be returned to nation states.”
Dutch government sources told EurActiv that they were unaware of the timing or content of the Cameron speech. "Anyone is welcome to make a speech in the Netherlands," said one source, adding: "We are curious about this, but it is a British thing."