BERLIN, March 6 (Xinhua) -- The German Federal Environmental Agency (UBA) called for the phased introduction of driving bans on heavily-polluting diesel vehicles on Tuesday.

Speaking to the newspaper Sueddeutsche Zeitung (SZ), UBA president Maria Krautzberger recommended the creation of two distinct "blue placards" which would govern vehicle access to German cities. Whether a car received a "light blue" or "dark blue placard" would hereby be determined by their nitrogen oxide emissions levels.

"This way the cities would have the possibility to react to their specific air quality circumstances", Krautzberger argued.

The UBA president proposed that technically upgraded diesel vehicles with "Euro 5" motor types and already registered cars with "Euro 6" motors could receive the light blue placard, while the newest and cleanest Euro 6d and Euro 6d-TEMP variants were awarded a dark-blue placard.

Krautzberger expressed optimism that the light blue placard would already ensure that air pollution levels in many cities were "back in line with EU regulations by 2020." The more restrictive dark blue placard could then be phased in-later and would be targeted specifically at cities with the highest nitrogen oxide emissions levels.

In a recent landmark case, the Federal Administrative Court in Leipzig ruled that cities in Germany can unilaterally impose driving bans on certain types of diesel vehicles to reduce nitrogen oxide pollution.

While such radical measures would have to be proportionate and phased in gradually, they were a legally-valid last resort for municipal authorities to remain within European Union (EU) clean air regulations.

The verdict by the Federal Administrative Court further complained there was no nation-wide legislative framework for driving bans, a circumstance which needed to change to prevent the emergence of a patchwork of different policies and standards across German cities.

Responding to such criticism, German Chancellor Angela Merkel (CDU) has since vowed to hold urgent consultations over the introduction of a "blue placard." However, Andreas Scheuer (CSU), the new designated transport minister of the next "grand coalition" government, has already announced that his ministry would resist any driving bans which he described as "objectively wrong.

Scheuer told the newspaper Passauer Neue Presse that although citizens were entitled to clean air, this did not justify the "quasi-expropriation" of diesel vehicle owners.

At the same time, a report by the magazine SPIEGEL warned on Tuesday that politicians were placing an inappropriate focus on removing older diesel vehicles from traffic.

SPIEGEL cited data from the UBA which showed that the bulk of urban air pollution was created by recent diesel vehicles under the "Euro 5" and "Euro 6" norms which were both more numerous on German streets and less efficient with regards to their nitrogen oxide emissions compared to older "Euro 4" variants.