The European Commission is taking the United Kingdom to court for failing to tackle illegal levels of air pollution.
The European Commission, the executive arm of the European Union, said it made a decision to refer Hungary, Italy and Romania to the Court of Justice over persistently high levels of particulate matter.
"The commission had to conclude that. that the additional measures proposed are not sufficient to comply with air quality standards as soon as possible, and therefore are being referred to court", Vella said.
The six are accused of failing to tackle nitrogen dioxide levels.
The six member states had failed to deliver "credible, effective and timely measures to reduce pollution as soon as possible, as required under European Union law", a statement from the commission said.
Environmentalists say toxic air results in more than 400,000 early deaths across Europe each year.
Germany, Britain and France were targeted for failing to meet limits on NO2 while Italy, Hungary and Romania exceeded limits on particulate matter.
"We will shortly build on our £3.5bn plan to tackle roadside emissions with a comprehensive Clean Air Strategy setting out a wide range of actions to reduce pollution from all sources".
Action against the Czech Republic, Slovakia and Spain has not been pursued as measures being put in place in each of the countries, "appear to be appropriate, if implemented" the Commission has claimed. The problem was declared a public health emergency by a cross-party committee of MPs in 2016.
Warning that legal action alone will not solve the pollution problem, Mr Vella also unveiled a raft of new Commission measures to help member states promote cleaner air. The Commission should have made no exceptions and referred them all.
The European legal case now moves to the ECJ, which will hold a hearing within months.
But she added that the process behind legal action should be "far more transparent" to allow citizens to know why some countries are taken to court and others not. If it does not, the court can then impose large fines.
It is unclear when the ECJ's jurisdiction over the UK's environmental concerns will end following Brexit. If a Brexit deal is agreed, Britain will be in the transition period and.
In a separate action, the commission proposed the first-ever Carbon dioxide emissions standards for new trucks travelling within the EU.
Reacting to the announcement, ClientEarth lawyer Ugo Taddei said: "We are glad that, at last, the Commission is taking serious steps to fight air pollution before the Court of Justice. Manufacturers that keep disregarding the law have to bear the consequences of their wrongdoing".