The revelation that the British government was assuring Libya that it did not want Abdelbaset al-Megrahi to die in jail came as a surprise to the Americans, who had been told the Lockerbie bomber would live out his full sentence in Scotland.

The disclosures are embarrassing for Gordon Brown, but most diplomats and observers on both sides of the Atlantic said the "special relationship" was safe despite the episode.

Instead corrosion to the relationship, which has always been far more special to London than to Washington, is actually more deep-seated and of a very different provenance. It is threatened by changes in the global strategic balance, embodied by the rise of China, India and other powers, which relentlessly diminish Britain's significance in the world.

The Megrahi case has caused a much bigger stir in Britain than in America. It drew a lot of US media attention on the day of his triumphal return to Libya but little since, apart from occasional coverage of the row in Whitehall.

The British embassy has received a few dozen items of hate mail – not much more than usual. The Scottish Tourist Board, which reported 340,000 US visitors last year, has received about 700 emails from Americans critical of its decision, accompanied by threats not to go to Scotland again. A web-based move to boycott oatcakes, whisky and other signature Scottish products gained little momentum.

The White House has largely ducked comment after its initial criticism of the release. "There's a reason for that. We want to draw a line under it," said one official.

"I get the impression that Hillary Clinton and the politicians in Washington were just going through the motions," said Tomas Valasek, the director of foreign policy and defence at the Centre for European Reform. "It would be very difficult for the US to accuse Britain of following a 'realist' approach, when Obama has made it a point of his foreign policy to work with countries with which there are serious differences."

On the campaign trail the Obama camp let it be known that he regarded Germany as being just as important as Britain and that his preference would be to work with Europe as a whole. But in the day-to-day business of foreign policy he has found that Europe rarely functions as a whole, while British support is critical both in Afghanistan and in the looming confrontation with Iran over its nuclear programme.

The UK receives a far bigger share of US intelligence – particularly electronic eavesdropping – than Israel, France or any other ally. British officials sit in on many high-level meetings in Washington not attended by other Europeans, and Britain has more privileged access to military technology than any other country. That will be even more true when a bilateral arms treaty, now slowly passing through the Senate, is ratified.

There are however, underlying long-term trends that are likely to dilute and perhaps eclipse the partnership.

Michael Rubin, an analyst at the conservative thinktank the American Enterprise Institute, argues that the relationship had been special primarily to a generation which fought the second world war together.

"As that group gets smaller and memory fades, it seems both countries are prioritising naked self-interest above common principle," Rubin said.

Paul Ingram, the executive director of the British American Security Information Council, said that while Britain was easily the most important US ally in Afghanistan its military role was now so dwarfed by the growing US presence that it was close to irrelevant. "If the British pulled out tomorrow it would be much more a political problem for Americans than a military one," he said.

When the US and Nato presence is finally wound down in Afghanistan, Britain could find its peacetime usefulness to Washington has eroded over the years.

"When you survey the world from the White House, Europe is still a small part of what the US sees, and Britain is just a small part of Europe," Valasek said. "People in Washington say we want to have a special relationship, but we need that relationship with Europe as a whole. We need more than Britain can provide."