Transparency campaigners have criticised Ireland and four other EU countries for using free Audi and BMW cars while negotiations were under way over strict new emissions targets.
The German luxury car manufacturers loaned scores of vehicles over a 36-month period that coincided with the debate over new limits for carbon dioxide emissions.
Ireland, Denmark, Cyprus, Lithuania and Greece held the EU presidency for six months each since January 2012.
The five countries were responsible for leading policy negotiations during their respective presidencies.
EU member states and companies sponsoring EU presidencies say sponsorship is an entirely legitimate practice, subject to strict rules, that dates back many years.
Critics say its continuance during the emissions debate - in which the car companies have an interest - was worrying.
"Of course, a direct conflict of interest is always hard to prove, but it can certainly be said that some German car producers have good timing when it comes to donations and sponsorships," said Rebecca Harms, co-president of the Greens.
Marcella Smyth, a spokeswoman for the Irish presidency, said that in an effort to deliver "a cost-effective presidency" a combination of contracts, including for the loaned cars, had saved a total of €1.4m.
She said Ireland had used public transport whenever possible and reserved the cars for driving ministers and other officials to and from airports.
Talks have now concluded on the proposed legislation, with a vote set for the European Parliament on Tuesday.
The parliament is expected to back a compromise deal, but the Greens say they will vote against it.
Transparency campaigners are also critical of the sponsorships.
"It is very worrying that governments, in return for relatively small amounts of sponsorship money, are willing to risk damaging their credibility and that of the EU presidency," said Olivier Hoedeman, research and campaigns coordinator at Corporate Europe Observatory, a Brussels corruption watchdog.
BMW and Audi say their sponsorship was only about raising their profile in European markets.
Audi said it regularly applied via tenders to provide cars for events because "it helps the Audi brand to increase both reputation and popularity".
EU member states say there is nothing wrong with accepting corporate sponsorship and several of them have proudly announced their relationship with BMW or Audi, mentioning the deal in press releases and publicity material.
Greece, which took over the presidency in January and has free use of 69 Audi cars, sent a message on Twitter declaring: "Audi is Grand Sponsor of the Hellenic EU Presidency.".
A spokesman for the European Council, which represents heads of state and government, said the council did not keep systematic records of sponsorship, but the practice of car companies providing cars for free dates back years and in the past has included non-German and German companies.
Although the new EU emissions rules are the toughest so far in the world, they are a dilution of the original proposal from the European Commission.
The commission officially published its proposal in July 2012, seeking an existing limit of 130 grams of carbon dioxideper kilometre (g/km) to be cut to 95 g/km by 2020.
An outline deal was first reached in June 2013, but that was torn up, paving the way in November last year for a weaker agreement.
Although the adjustments are relatively minor, they are enough to give breathing space to the carmakers.