Pressure is mounting on Jean-Claude Juncker, the European commission president, after fresh allegations emerged outlining the tactics he used when he was prime minister of Luxembourg to promote the country as the destination for multinational corporations.

Bob Comfort, the former head of tax for Amazon, claimed Juncker had fiercely courted the online giant, behaving as a “business partner” and “helping solve problems”. Months after arriving in Luxembourg in 2003, Amazon secured a confidential deal from the local tax office. Two months ago that deal became the subject of a formal investigation by the European commission.

In an interview with the Luxembourg newspaper d’Lëtzebuerger Land, Comfort said: “The Luxembourg government presents itself as business partner, and I think it’s an accurate description: it helps to solve problems.”

He recalled meetings with top civil servants from the finance ministry and Juncker, who was then serving as both prime minister and finance minister. “His message was simply: ‘If you encounter problems which you don’t seem to be able to resolve, please come back and tell me. I’ll try to help.’”

Shortly before he retired from Amazon this summer, Comfort took on the additional role of Luxembourg’s “honorary consul for the Seattle region”. An official announcement noted he had “a long-standing relationship with Luxembourg”.

The mounting controversy around Luxembourg’s accommodating approach to multinationals’ tax planning came as George Osborne revealed details of a new “diverted profits tax”. The new tax, also called the Google tax, was announced in the autumn statement last week and the chancellor has claimed it will raise £1bn in the next five years. The new measures will target companies shifting profits out of the UK in artificial ways, with a punitive 25% tax rate from April next year.

A Treasury source said: “HMRC already has a pretty good idea of the companies that use these kind of structures and will be issuing notices to those companies. Companies then have 30 days to dispute the payment but if HMRC believes they are using the structure they would have to pay immediately. Obviously there may be companies that may not be on HMRC’s radar. We would expect them to report to HMRC that they are using these structures. If they don’t, that would be illegal.”

The Treasury expects the tax to hit companies such as Amazon and Google that make billions of pounds from sales to UK customers but book the revenues in companies overseas.

Margaret Hodge, chair of parliament’s public accounts committee, said: “This is an important symbolic statement but it is really on the margins … what you really need is international agreement and a really, really tough, determined HMRC.”

Pressure is building across Europe for tougher, quicker action. European commission investigators have said they believe the 2003 deal with Luxembourg, which is still in use by Amazon – appears to be so generous as to amount to illegal state aid, though inquiries are ongoing.

Amazon EU Sarl, the company with which customers across much of Europe do business when they buy online from the retailer, took €13.6bn (£10.7bn) in sales last year, up from €11.9bn in 2012. The company’s taxable profits were reduced, however, after deductions for royalty payments to another Luxembourg unit, an Amazon-controlled limited partnership, which is not subject to tax in the Grand Duchy.

In a statement to the Guardian, the retailer said: “Amazon has received no special tax treatment from Luxembourg – we are subject to the same tax laws as other companies operating here.”

Last month, just after the commission’s state aid inquiry was revealed, the International Consortium of Investigative Journalists, together with the Guardian and a team of international journalists in more than 20 countries, published investigations into a cache of hundreds of leaked tax deals secured by multinationals with Luxembourg authorities. Many of the deals rubber-stamped complex and aggressive tax avoidance structures that drained the tax coffers of other countries.

This week, a second batch of leaked documents has shed light on tax deals secured for Skype, Koch Industries and others. The widening scandal has led to suggestions that Luxembourg, rather than offering selected multinationals sweetheart deals in contravention of state aid rules, had been engaged in facilitating tax avoidance on an industrial scale.

Juncker has denied the charge. “I am not the architect of the Luxembourg model because this model doesn’t exist,” he said last month, insisting that his government did no more than compete hard for inward investment as others did.

Comfort, who retired from Amazon this year, recalls touring almost every country in western Europe with a small team looking for a regional base in 2003. “The final decision was made at the highest level of the company,” he said. “[Tax] did play a significant role, but it wasn’t the overriding factor.”

Journalist Brad Stone, in his book The Everything Store, described Comfort as the “true architect” of Amazon’s “arcane tax structure … deploying every trick in the book and inventing many new ones”.

Comfort says Amazon did nothing out of the ordinary by locating its European operation in Luxembourg, and has played down the significance of allegations it is not paying sufficient tax. “Governments think they aren’t getting their fair share, but multinationals follow the existing rules of the bilateral treaty system. The government’s unhappiness relates to an arguably outdated legal framework that has been designed for the 20th century.”

Asked what his role as honorary consul for the Seattle region entailed, Comfort said: “I think there are only four Luxembourgers in all of Washington State. So I am rarely called upon to perform consular functions such as helping with visas or bailing people out of jail. My work here is to help attract business to Luxembourg.”

Comfort could not be reached for further comment.