The company - founded by the pro-Brexit entrepreneur Sir James Dyson - first went to court over the issue in 2013, before new European Commission laws governing efficiency tests took effect.
It argued at Europe's General Court that tests carried out on empty machines were misleading as they do not reflect real-life use, with some manufacturers even cheating the system in a manner similar to that witnessed during the so-called dieselgate scandal.
The maker of bagless vacuums made the case for performance to be tested in real-world conditions, saying models using bags and filters can become clogged and claimed some were rigged to cheat the testing regime.
Dyson alleged that cleaners with an A-rated efficiency label could soon drop to a G grade once it is used.
The decision followed several hearings, including a previous defeat at the General Court in 2015, and a separate ruling at the European Court of Justice during the summer that rejected Dyson's demand that labelling should contain more information on the testing.
The current labelling system will remain in place until early in the new year pending the possibility of an appeal by the commission.
A spokeswoman for the company welcomed the ruling and said: "We have been arguing consistently that the commission committed two legal violations to the detriment of European consumers and Dyson.
"The lab tests for the energy label do not reflect real use as EU law requires they must, and the EU label flagrantly discriminated against a specific technology - Dyson's patented cyclone.
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"This benefited traditional, predominantly German, manufacturers who lobbied senior commission officials.
"Some manufacturers have actively exploited the regulation by using low motor power when in the test state, but then using technology to increase motor power automatically when the machine fills with dust - thus appearing more efficient.
"This defeat software allows them to circumvent the spirit of the regulation, which the European Court considers to be acceptable because it complies with the letter of the law.
"In these days of Dieselgate, it is essential consumers can trust what manufacturers say about their products.
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"But the Commission endorsed a measure that allowed Dyson competitors to game the system.
"The legal process has been long, distracting and expensive, with the odds stacked against us. Most businesses simply do not have the resources to fight regulations of this nature. It is appalling that this illegal and fundamentally anti-competitive behaviour has been endorsed for so long."