Greece's top administrative court this month ruled that approval by Greek authorities of the investment plan, which included expanding an existing cruise terminal, was deficient and had to be amended, a source at Piraeus Port Authority (PPA), the port's operator, told Reuters.

PPA did not elaborate, but said the ruling caused the "temporary pausing" of construction work on the cruise terminal.

It added it was committed to implementing its investment in Greece, including any possible amendments, and has been closely working with authorities for "the best possible outcome".

COSCO bought a 51% stake in Piraeus Port under a 2016 privatisation deal and last year acquired an additional 16% stake, although it had not fulfilled a key condition of concluding mandatory investments worth about 294 million euros.

The company blamed delays beyond its control for that failure and Athens allowed it to raise its stake by amending the 2016 deal, arguing that any other option would stop investments and damage the Greek economy.

Piraeus has become a major transhipment hub for goods destined for Mediterranean and Black Sea ports since COSCO took over its container operations in 2009 and upgraded its infrastructure.

(Reporting by Angeliki Koutantou; Editing by Alexander Smith)