The EU’s top environmental official alerted his colleagues that motor manufacturers were gaming European emissions tests more than two years before US authorities uncovered widespread cheating by Volkswagen, according to internal European Commission documents obtained by the Financial Times.

Despite the warning sounded by Janez Potocnik, the then EU environment commissioner, Brussels did not take swift action to crack down on the practice but instead left in place an earlier plan that allowed the emissions loopholes exploited by Volkswagen to remain through to 2017.

Volkswagen has now suspended about 10 senior executives as part of its inquiry into the crippling emissions scandal that has rocked Europe’s biggest carmaker. Wolfsburg-based VW last month admitted, in response to the US investigation, to using an illegal piece of software in its diesel engines to cheat in tests for dangerous nitrogen oxide emissions.

The scandal has raised broader issues about how regulators in the EU ensure compliance with legal limits on such emissions.

In negotiations this month, the commission and EU nations considered giving car manufacturers another two years to fully comply with more robust tests that would force cars to really meet EU limits on NOx emissions, allowing the requirements to be phased in through 2019.

The documents obtained by the FT show the manipulation of emissions testing by carmakers was widely known — and hotly debated — at the EU’s highest levels much earlier than previously believed.

In a letter to Antonio Tajani, the European commissioner in charge of industrial policy, written in February 2013, Mr Potocnik said ministers from several EU countries believed the “significant discrepancy” between how cars performed in the real world compared with in the testing laboratory was the “primary reason” air quality standards were not falling to levels required by EU law.

“There are widespread concerns that performance [of cars] has been tailored tightly to compliance with the test cycle in disregard of the dramatic increase in emissions outside that narrow scope,” Mr Potocnik wrote. Cars are required to comply with EU emissions limits “in normal driving conditions”, he wrote.

“My services and I are often put in an uncomfortable position when defending the perceived lack of action by the commission and member states in addressing the obvious failure to ensure this.”

He urged Mr Tajani to quickly propose new measures to strike back at carmakers, such as withdrawing emissions approvals for entire model lines and requiring “remedial action” from manufacturers.

People involved in the talks said concerns at the time centred on legal techniques used by manufacturers to improve their results, rather than on the possibility carmakers might be breaking a 2007 EU law using software, known as “defeat devices”, that VW has admitted using.

At the centre of the internal commission wrangling was the discrepancy between NOx emissions recorded in laboratory tests conducted by regulatory authorities and the much higher levels detected during road tests.

The discrepancies were highlighted as early as 2011 by the commission’s joint research centre . Those same discrepancies, when uncovered by US investigators this year, led the US Environmental Protection Agency to confront VW directly, leading to the admission it had been cheating.

The exchange of letters between Mr Potocnik and Mr Tajani was prompted by correspondence that both received in January 2013 from Ida Auken, the then Danish environment minister. Ms Auken criticised policy plans published by the commission in November 2012 that delayed on-road emissions tests until 2017. She said in her letter that this timeframe was “unacceptable” and the commission should “act upon this critical situation as soon as possible”.

The exchange of letters is the clearest indication to date that there were concerns within the EU’s leadership over how the bloc was responding to the JRC’s findings well before the VW scandal burst into the open this year.

Mr Potocnik, who left the commission at the end of his term in November last year, declined to comment. But Mr Tajani, now a vice-president of the European Parliament, who was in charge of overseeing NOx regulation as industry commissioner until last year, insisted the efforts he made to set out a plan for introducing “real driving emissions” tests show a determination to tackle the issue.

“I have not refrained from ensuring that the issue of real drive emissions is dealt with and followed up appropriately, in close co-operation with my fellow commissioner Potocnik and our respective services,” Mr Tajani said.

In a written response to Mr Potocnik in March 2013, also obtained by the FT, Mr Tajani emphasised that the introduction of tests reflecting real-world emissions “should not be delayed further” beyond 2017.

But in a joint response to Ms Auken, both Mr Potocnik and Mr Tajani largely stood by the commission’s 2017 date, arguing that “in many cases a significant redesign of diesel vehicles will be required” to meet emissions limits in the real world. Still, they said officials were examining whether a faster timeframe was possible.

The commission has come under intense criticism for succumbing to carmaker lobbying to water down and delay implementation of on-road tests that may have prevented the Volkswagen scandal.

But in an interview with the FT, Ms Auken said Brussels’ foot-dragging arose primarily from the economic crisis that had gripped the eurozone, forcing EU officials to prioritise pro-industry policies in a desperate attempt to boost growth.

“There was a general perspective in the commission that the crisis was the priority, that there were problems enough, let’s not create new ones,” said Ms Auken, who lobbied hard for the EU to take more action and is now a Danish MP.

“Everything was about the financial crisis and saving the euro … and in that situation, you have to be very precise which battles to fight.”

While Mr Potocnik declined to be interviewed for this story, in the immediate aftermath of the VW scandal his frustration was visible on Twitter, where he commented: “Do we really always need scandals and disasters to do the right thing?”

People involved in the emissions debate said Mr Potocnik raised concerns about carmakers misleading testers as early as May 2011, when he warned in a speech that “some cars may be ‘tweaked’ to fulfil the required test cycle in laboratory conditions but run outside the optimum when they are on the road”.

According to people involved in a closed-door gathering in 2012 that included EU officials, the car industry and environmental groups, to thrash out plans for the future regulation of the sector, Mr Potocnik told attendees flaws in the emissions testing of diesel cars meant consumers were being grossly misled.