The ruling by the Council of State, Greece's top administrative court, came six days after the sudden closure that triggered national outrage and destabilised the coalition government.
It was announced after Prime Minister Antonis Samaras himself proposed a temporary reopening until a government move to form a new state broadcaster can be approved by parliament.
Samaras also Monday promised the two parties with which he shares power, and which had bitterly opposed ERT's closure as part of an unpopular austerity drive, that he would reshuffle his cabinet at the end of the month.
The prime minister's allies had urged him to back down, warning that the coalition could collapse if he persists with the ERT cull that sparked an uproar at home and criticism from abroad.
News of the court ruling was greeted by thunderous applause by thousands of people gathered outside ERT headquarters in a northern suburb of Athens.
ERT staffers have occupied the broadcaster's offices in Athens, Thessaloniki and other major cities since the closure, and kept up a rogue broadcast on the Internet, with assistance from the European Broadcasting Union and Greek radio stations.
Samaras' New Democracy party claims ERT ate up €300m (US$400m) a year for an overall viewer rating of 4%, less than half of its private competitors.
The government said last on Tuesday as it shut down ERT that the broadcaster was "a case of an exceptional lack of transparency and incredible extravagance".
It has offered to compensate ERT's almost 2,700 employees while pledging to set up a new public broadcaster within months with less than half the original staffing levels.
Also on Monday, the main opposition party Syriza staged a rally on central Syntagma Square to demand early elections.
Credit ratings agency Moody's meanwhile warned that "fraying political consensus" and "slippage" in its privatisation programme could cost Greece another downgrade.
Persa Zeri of Athens' Pantion political science university, said the conflict within the governing coalition showed that "a mentality of cronyism" persists there, especially concerning the public sector.
"No one wants to change anything," she said. "The ERT was an unmanageable body, it's true that it had to be shut down, but you can't close it without an agreement within the coalition. There has to be a democratic dialogue."
The three-party coalition was already a delicate balancing act, with the socialists and moderate leftists forced to accept additional job cuts to safeguard Greece's massive EU-IMF loan lifeline.
Three separate opinion polls on Sunday showed that Greeks rejected ERT's closure by between around 64 and 68%, though a majority also opposed elections.
Samaras on Sunday accused his coalition partners of "hypocrisy", arguing that all three coalition party heads had agreed to chop 2 000 public-sector jobs to help keep Greece afloat.
"From where should we cut these jobs apart from the undeserving ERT... one of the bastions of obscurity and privileges?" Samaras told a party conference.
ERT's radio broadcasts began in 1938 and its television programming in 1966.
Source: AFP, via I-Net Bridge
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