The files, obtained by the CIA after the fall of the Berlin Wall, name Britons who spied for East Germany in cold war

Britain has been accused of "sheltering communists" after refusing to hand over a cache of Stasi files revealing the names of British spies who worked for the East German secret intelligence agency during the cold war.

The cache belongs to a set of mysterious microfilm images, known as the Rosenholz (Rosewood) records, that contain 280,000 files giving basic information on employees of the foreign intelligence arm of the former GDR.

The records were obtained by the CIA in murky circumstances shortly after the fall of the Berlin Wall. American agents analysed the data before distributing relevant portions to countries in which the Stasi were active.

A swath of files relating to Stasi activity in the UK were given to MI5 by the Americans in the 1990s. Now Germany wants the files back, to add to its extensive archives on the GDR's ministry for state security, commonly known as the Stasi.

If the files are returned to Germany, they will be made available, unredacted, to scholars and historians. That means that British Stasi sympathisers and spies could be outed for the first time.

Today, Germany only has those sections of the Rosenholz discs pertaining to activity in former West Germany – though the governments of Norway, Denmark and Sweden recently indicated they were ready to hand over the Rosenholz files they were given by the CIA more than 10 years ago.

Since the return to Berlin of the West German portion of the Rosenholz files in 2003, a number of public figures have been outed as Stasi collaborators, most recently a priest who allegedly spied on Joseph Ratzinger, now Pope Benedict XVI .

"We need access to these British files in order to understand the cold war, which was a war fought by secret intelligence operatives all over the world," said Helmut Müller-Enbergs, one of the world's leading scholars on the Stasi.

With fellow academics, he is demanding that Britain return the Rosenholz files to the Stasi archives in Berlin. "Given that the Brits have long been considered world class in intelligence gathering, it is especially important for us to understand how the Stasi was able to operate in the UK."

"The UK is not a country known for sheltering communists, so why then will they not reveal to us who in Great Britain was working for a communist regime?" said Müller-Enbergs, a researcher at the Stasi archives in Berlin (BStU) and visiting professor at Gotland University, Sweden.

Roland Jahn, the federal commissioner for the Stasi archive, said: "These records could offer an important complement to those Stasi files we already have, and thus make an important contribution to the reappraisal of the role of East German state security in Europe."

The Stasi archives already encompass 69 miles (111km) of files, including 39m index cards, 1.4m photos and 34,000 video and audio recordings. But the Rosenholz files are key because of the systematic and deliberate destruction of most of the records relating to a Stasi division known as the Hauptverwaltung A (HVA), which was responsible for running an extensive network of spies in the west.

When the Berlin Wall fell in November 1989, a high level committee agreed (with the blessing of the West German chancellor Helmut Kohl) that the HVA archives should be destroyed – a decision described by Die Zeit recently as one of the worst mistakes made during reunification .

The microfilmed files obtained by the CIA – in what the Americans described as a "clandestine operation" which may have included a pay-off to a rogue KGB agent – are the key because they contain copies of the card indexes of the HVA, listing the real names of all the agents, informers and targets of the Stasi's foreign operations.

Put together with files already in the BStU's possession, they allow scholars to build up a picture of who the spies were, who they were spying on and how the Stasi carried out missions abroad.

Herbert Ziehm, deputy head of the disclosure/information division of the BStU, said it would be "lovely" for Britain to return their portion of the Rosenholz files. "Then we would be able to see exactly who was spying for the Stasi in Britain – from other sources we already know what information they were delivering, but this would enable us to work out who they were," he said.

Ziehm was part of the negotiating team which persuaded the US to hand over the Rosenholz discs to Germany's Stasi archives in 2003.

Even just getting those Rosenholz files pertaining to east and west was a drawn-out process, he said: "The negotiations took a number of years. "The Americans were reluctant to co-operate for some time.One CIA agent put it like this: when you get some loot from a mission, you don't share it." Ziehm believes the CIA obtained the files in 1992 "at the very latest".

Ziehm said the files are important in puzzling how the Stasi operated abroad. "We already had three-quarters of the information – Rosenholz gives us the opportunity to gain the missing quarter," he said.

Thomas Wegener Friis, an associate professor at the Centre for Cold War Studies at the University of Southern Denmark, said the return of the files was about transparency rather than naming and shaming.

"It's not just a question of outing people – though we should not be shy to name those who worked for the Stasi abroad," he said. "More important is being able to understand how intelligence agencies worked on an operational level during the Cold War. It will allow us to learn lessons for the future."Asked by the Guardian why Britain refused to hand over the Rosenholz files, the Foreign Office, which handles press requests for MI5 and MI6, said: "We don't comment on intelligence matters."

No Briton has ever been prosecuted in the UK for spying for East Germany, according to Anthony Glees, professor of politics at the University of Buckingham and director of its Centre for Security and Intelligence Studies.

In 1999, the then home secretary, Jack Straw, told MPs that MI5 was investigating more than 100 Britons suspected of having been Stasi agents.