BRUSSELS, Jan 11 (Reuters) - The European Commission received a letter from the Polish government on Warsaw's contentious judicial reforms, a spokesman said on Tuesday.

Jan.11 marked a deadline for the nationalist government in Poland to inform the Brussels-based Commission of when and how exactly it plans to dismantle a disciplinary system for judges, which should have been frozen on the order of the top EU court.

Warsaw refused to heed the decision by the Luxembourg-based European Court of Justice, which hence approved fines of 1 million euros a day. By the end of Monday, Warsaw had time to explain how it would align itself with the ECJ, or pay up.

The spokesman said the Commission was now analysing the reply and would react "swiftly".

A source at the European Commission, speaking to Reuters separately and under condition of anonymity, said the letter was in Polish so it would first need to be translated.

Should it fail to satisfy the Commission - which acts as the enforcer of joint EU laws, including on safeguarding the judiciary from political meddling - the person said it would send an invoice to Warsaw, with a 45-day deadline to pay. (Reporting by Gabriela Baczynska)