The American Bar Association (ABA) has joined a campaign against the European Commission's planned changes to the competition law system.
The changes would see more competition law issues handled by national authorities to help relieve the enormous workload on the Commission under the existing system.
But the ABA believes that changing the system would fragment the single market and hold up cross-border deals.
Oppositions to the proposals to make Article 81(3) directly applicable have already been voiced by Unice, the Confederation of European Employers Groups, the Confederation of British Industry and other business leaders.
The ABA Section of Antitrust Law has delivered a strongly-worded document to the Commission stating that while it supports the general proposal to make the Article directly applicable, it finds the proposal "inefficient in and of itself".
The document adds that if the proposed decentralisation from the EU to national authorities is not clarified, it "could have significant adverse effects on EU competition law and enforcement, particularly in relation to transparency, coherency and legal certainty".
The ABA argues that the proposal could lead to inconsistent application of law among member states, authorities and the courts.
It has called for the Commission to improve training, coordination, and understanding of competition law.