BRUSSELS (Reuters) - The European Commission is expected on Thursday to threaten Britain, Belgium and Luxembourg with potential legal action over sales of Daimler's (>> Daimler AG) cars which the executive says are in breach of EU rules on coolants, an EU source said.

The Commission will also launch legal proceedings against Germany over Daimler's refusal to remove a banned refrigerant from new cars, the source said, speaking on condition of anonymity.

Officials have for months been investigating the German luxury car maker's refusal, backed by Berlin, to follow an EU law banning the air-conditioning coolant known as R134a from the start of last year.

The carmaker insists its refusal to phase out R134a, a global warming agent more than 1,000 times more potent than carbon dioxide, is justified by safety concerns.

The only available replacement, Honeywell's (>> Honeywell International Inc.) R1234yf has a global warming potential only four times more than carbon dioxide but Daimler says it can emit toxic hydrogen fluoride gas when it burns.

A source told Reuters that the Commission suspected the three countries circumvented EU rules that say new car models should use more environmentally-friendly coolant by approving new models as old ones.

"On Thursday, the Commission will send pilot letters to the UK, Belgium and Luxembourg for the practice of extending old vehicle approvals to new models applied by their competent authorities," the source said.

"The Commission suspects that these practices are meant to circumvent the MAC (Mobile air condition systems) directive."

By sending the letters, the Commission will be seeking clarification from the three countries on their compliance with EU rules.

Pilot letters are part of a procedure that precedes official legal action from the Commission.

After safety tests, other European car makers have switched their newest models to the coolant developed by Honeywell in partnership with Dupont (>> E I Du Pont De Nemours And Co).

Under the EU infringement system, countries found not to be enforcing the bloc's law can be taken to the European Union's top courts in Luxembourg, which have the power to impose fines.

(Writing by Justyna Pawlak, editing by David Evans)