LONDON — Four years ago, European auto regulators identified and debated a loophole in their pollution testing rules that could render vehicle emissions tests ineffectual, but it was ultimately left in place, internal documents show.

The loophole lets carmakers change the performance settings of their engines before a pollution test. “A manufacturer could specify a special setting that is not normally used for everyday driving,” British regulators warned, according to minutes of a 2011 meeting in Geneva of officials across the region.

The debate throws new light on Volkswagen’s costly emissions cheating — particularly the question of whether VW violated European testing rules.

In September, Volkswagen admitted to installing “defeat device” software in 11 million cars, programming them to detect when they were being tested for nitrogen oxide emissions. The result was a substantial reduction in emissions. Volkswagen made the admission, after deceiving regulators in the United States for several months, in the face of an inquiry.