Downing Street has been laughing off a series of jibes from grandees ahead of David Cameron's speech on Europe on the grounds that few have had the ear of prime ministers and presidents in recent years.
No 10 may find it less easy to dismiss an intervention by a new recruit to the grandees' club who, until last year, was at the heart of Britain's transatlantic relationship for more than a decade.
If Tony Blair's famous Atlantic bridge, in which Britain serves as a link between Europe and the US, ever took human form then Sir Nigel Sheinwald carried quite a burden as British ambassador to the EU and Washington. Bearing the scars, or colours, from 10 years on the transatlantic bridge Sheinwald delivers one key message before the prime minister's long-awaited speech on Europe on Friday. Britain will diminish its influence in the US and even jeopardise its position as the investors' "gateway" into the EU's single market if it raises questions about its commitment to the EU, let alone leaves it altogether.
"A Britain on the sidelines of Europe, even more out of Europe, would not be in the American interest and would not be in the interest of other major European partners," Sheinwald told the Guardian from New York during a business trip on Tuesday.
The remarks by Sheinwald are a significant contribution to the debate because they echo, almost word for word, a warning by the Obama administration of the dangers in Cameron's strategy to be outlined in the Netherlands on Friday. Philip Gordon, the US assistant secretary of state, questioned the prime minister's plans for a referendum and said that "a strong British voice" in the EU was "in the American interest".
Sheinwald said he was not in the slightest bit surprised by the US intervention which was the result of a "conscious decision".
"The US view is a long standing American position. They did not think it was necessary to say so on a consistent basis because the issue has not arisen in the UK in the way it has in the past few months. There is a fear that by a series of decisions, negotiations and conjunctures you might be on a path to asking the British public to vote in a way that they would not find palatable."
Eurosceptics, who dismissed the remarks by Gordon on the grounds that he cannot know what constitutes the British national interest, will now have to contend with the views of the man who eased the path for Cameron's successful trip to Washington last March. This took place two months after Sheinwald retired as ambassador to Washington after a five-year stint that followed seven years at the heart of EU policy, as Britain's "permanent representative" in Brussels between 2000-03 and as Tony Blair's main European adviser between 2003-07.
One of the most arduous tasks of the "perm reps" is to attend the weekly meetings of all 27 ambassadors to the EU, known to anoraks as COREPER II, which gave Sheinwald an insight into the possibilities – and limitations – of EU negotiations. The prime minister is expected to pledge to hold a referendum on a "new settlement" in Britain's place in the EU if he wins the next general election. Cameron would use Britain's veto in a major revision of the Lisbon treaty, designed to underpin new governance arrangements for the eurozone after the 2014 European parliamentary elections, to demand a repatriation of powers.
Sheinwald says Britain can secure special conditions as long as its partners believe it is acting in a cooperative manner. "The willingness of our partners to cooperate depends on a range of factors. But above all their perception of their own economic interests and whether it will help the single market in their minds or encourage the fragmentation of the EU which would be a bridge too far. It just depends what we ask them."
But Sheinwald says Cameron should not assume there will be a big-bang treaty renegotiation that will enable him to table extensive demands for the repatriation of British powers. "I still don't believe that that process is going to be anything other than a slow, difficult and gradual one given the complexity of the issues, the fact that we are still nation states and public opinion in most of the eurozone countries."
In his lengthy Foreign Office career Sheinwald, 59, witnessed the painful birth of the greatest example of European integration which is at the heart of the continent's current pain. He was a senior official in the foreign office's EU section in 1991 during the Maastricht treaty negotiations, which gave birth to the single currency, and was Europe Director in 1999 when the euro came into force and "perm rep" in Brussels in 2002 when its coins came into circulation.
Sheinwald takes issue with one of the prime minister's central calculations about the euro which is informing No 10's entire European strategy – that the "remorseless logic" of monetary union will lead to a fiscal union which Britain should encourage but not join. It is in those negotiations that the prime minister hopes to repatriate powers.
"I am not disagreeing that there is a remorseless logic about further integration in the eurozone," Sheinwald says. "What I am arguing about is the matter of degree – the sense that it is in our interests to make them as separate as possible from us is where I would part company from the viewpoint."
Sheinwald laughs as he sympathises with Lord Kerr of Kinlochard, one of the few people also to have served as ambassador to Washington and the EU, who warned in the Guardian last month that the prime minister is in danger of leaving the EU by accident. He adds to the warning of his former boss by saying what would happen if the British people voted no in the referendum.
"Whether that would be the be all and end all – or whether as in Ireland or in Denmark you would have a re-run of the referendum – who knows? But it would create a major crisis in the British relationship with the rest of EU."
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