The British used to be past masters in the art of trading favours to secure their interests in the European Union.

But since 2010, Prime Minister David Cameron's government has largely neglected the practice to avoid upsetting sceptics in his ruling Conservative party who resent Britain's loss of sovereignty to the EU. Partly as a result, London's influence in the corridors of Brussels has diminished.

Cameron may pay a high price in the coming weeks when he needs to call in all the favours he can to achieve a successful outcome to his renegotiation of Britain's membership terms before holding a referendum on whether to stay in the bloc.

"All along, Cameron has not understood that solidarity matters," said John Kerr, who was Britain's EU ambassador in the crucial 1990-95 period when London secured opt-outs from the planned single European currency and from EU social policies.

Kerr, now a non-party member of the unelected upper House of Lords, told Reuters that then-prime minister John Major gave him leeway to explore deals with European partners and let him talk ministers into adapting their policy to secure higher goals.

That is how the Brussels game is played. Countries build allies by being helpful to each other on issues where they don't have a dog in the fight, and expect a return when their vital interests are at stake.

"No one did this better than the British," said Poul Skytte Christoffersen, who as Denmark's envoy in 1995-2003 brought complex negotiations on the EU's eastward enlargement to a successful conclusion.

"In the old days, the UK had a well-oiled and very co-ordinated machine. You could always count on the fact that when their representative said something, he had backing in council and some margin to adapt."

But over the past five years, Britain has cold-shouldered partners during the euro crisis and preferred to be outvoted on more EU decisions than in the past rather than joining compromises.

Cameron refused to pay a penny towards bailouts of euro zone countries, tried to block an EU fiscal pact at the height of the euro crisis to demand veto powers to protect London's financial industry, and declined to take in any of the Syrians stranded on Europe's highways - it is taking only a small number directly from Middle East camps.

Now, there are some signs that Cameron may be trying to build goodwill, beyond touring EU capitals.

After Islamist militants killed 130 people in attacks in Paris, he joined 71,000 fans in singing France's Marseillaise national anthem - not a natural entry on the Conservative party hymn sheet - at a soccer match in London. That earned gratitude in Paris.

Britain was first to write a cheque when the EU agreed to pony up three billion euros ($A4.43 billion) to help Turkey keep refugees on its soil - a helpful gesture to German Chancellor Angela Merkel, who is bearing the brunt of the refugee influx.

And London changed its policy to vote with Germany to water down EU vehicle emissions standards at a time when Volkswagen, powerhouse of the German car industry, was in deep trouble for using defeat software to cheat exhaust tests.

British officials dispute that it was a down-payment for Merkel's help in the renegotiation, but environmental activists are not alone in harbouring that suspicion.

Cameron told his party in October the talks would be "bloody hard work" and European Council President Donald Tusk, who is overseeing them on the EU side, said it would "very, very tough" to reach a deal.

In his day, Kerr made deals in the EU's council of permanent representatives (COREPER) during the unification of Germany, the Maastricht treaty on economic and monetary union, and the GATT global trade talks.

"In my five years in COREPER, Britain was only outvoted once, and that was a put-up job where we and the Germans agreed secretly to be outvoted so the French could claim a victory on the cultural exception in the GATT talks," he told Reuters.

"In return, the French let themselves be outvoted on the agricultural provisions that their farmers didn't like."

That kind of horse-trading is typical of the way EU members exchange favours, sometimes on unrelated dossiers.

Now Britain needs friends in Poland and other central European states whose workers have surged into its labour market over the past decade, fuelling hostility among Eurosceptics who say the country has lost control of its borders. In perhaps his most difficult renegotiation demand, Cameron is seeking to deny new EU migrants welfare and in-work benefits for four years - a challenge to the EU principle of non-discrimination.

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