The president of the European commission, Jean-Claude Juncker, spent years in his previous role as Luxembourg’s prime minister secretly blocking EU efforts to tackle tax avoidance by multinational corporations, leaked documents reveal.

Years’ worth of confidential German diplomatic cables provide a candid account of Luxembourg’s obstructive manoeuvres inside one of Brussels’ most secretive committees.

The code of conduct group on business taxation was set up almost 19 years ago to prevent member states from being played off against one another by increasingly powerful multinational businesses, eager to shift profits across borders and avoid tax.

Little has been known until now about the workings of the committee, which has been meeting since 1998, after member states agreed a code of conduct on tax policies and pledged not to engage in “harmful competition” with one another.

However, the leaked cables reveal how a small handful of countries have used their seats on the committee to frustrate concerted EU action and protect their own tax regimes.

Efforts by a majority of member states to curb aggressive tax planning and to rein in predatory tax policies were regularly delayed, diluted or derailed by the actions of a few of the EU’s smallest members, frequently led by Luxembourg.

The leaked papers, shared with the Guardian and the International Consortium of Investigative Journalists by the German radio group NDR, are highly embarrassing for Juncker, who served as Luxembourg’s prime minister from 1995 until the end of 2013. During that period he also acted as finance and treasury minister, taking a close interest in tax policy.

Despite having a population of just 560,000, Luxembourg was able to resist widely supported EU tax reforms, its dissenting voice often backed only by that of the Netherlands.

Among proposals popular in the code of conduct committee but opposed by Luxembourg were:

• Plans for tax authorities in each member state to subject their dealings with multinational businesses to peer review.
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• An investigation into cross-border tax avoidance strategies, known as “hybrid mismatches”, often used by multinationals to conjure up artificial tax savings.

• Improved information sharing between member states on tax deals granted to multinationals in private.

A spokesperson for Luxembourg’s finance ministry refused to comment on the positions previous governments had taken in private EU discussions. “We have no knowledge of the communications you claim to have, and whether they are genuine, and therefore cannot comment on them,” he said.

The spokesperson added: “In recent years Luxembourg has been at the forefront of the global trend towards greater transparency in tax matters and the fight against harmful tax competition.”

The Guardian spoke to another former member of the code of conduct committee, who did not want to be named but corroborated claims in the leaked cables that Luxembourg was regularly among those looking to frustrate EU efforts to tackle tax avoidance.