They are uniting to overturn a proposal to remove the so-called "country of origin" principle from upcoming EU legislation on unfair trading.
If the proposed directive on unfair commercial practices goes ahead in its present form, companies would no longer be covered by current regulations under which their advertising needs only to comply with the laws of the country where it was originated.
Now the EU consumer protection minister, David Byrne, wants to sweep away the principal established in October 2001. The result could be that UK advertisers would have to conform to 24 other sets of national law.
The plan was this week condemned by the Advertising Association, which claimed it made "a mockery of the drive towards an internal market".
Meanwhile, the European Association of Communication Agencies is writing to all its affiliates urging them to put pressure on their EU representatives to get the plan derailed.
Industry leaders accuse Byrne, who originally advocated the "country of origin" principle, of being prepared to ditch it in order to ensure the safe passage of the directive through the EU Council.
A significant number of EU countries have shown either outright or serious opposition to the internal market aspects of the proposed measure. Only Britain and three other member states continue to support the "country of origin" clause.
Industry efforts to protect the clause is intensifying in the run-up to a meeting of EU competitiveness ministers on Tuesday. If they agree, the measure will return to the European Parliament for a Second Reading.
Andrew Brown, the AA's director-general, said: "The directive must provide legal certainty for business through a strong 'country of origin' control mechanism. Anything less would be unacceptable to the UK ad industry."
He added: "The AA continues to support the UK government in its commitment to the cause of the internal market to achieve the agreed goals of improving competitiveness by removing barriers to the free movement of services."
Phil Murphy, the AA's head of European public affairs, warned that attempts by Brussels legislators to reach agreement on the directive at any price could prove costly to the whole ad business. The European Commission now had to prove that four years of discussions on the issue were not just "idle rhetoric", he added.
However, industry leaders argue that the "country of origin" principle is enshrined in EU law, which the directive has no power to change.