In an embarrassing move for President François Hollande's government, the Council of State has ordered it to issue a decree fixing a date for the tax to start being levied within six months, warning that a failure to do so would be illegal.
It points out that a law voted through parliament in August 2014, which declared the tax would be up and running on 31 December, was suspended shortly before that date but has not yet been repealed.
Environmental campaign Alsace Nature took the case to the body, which ruled the government is "charged with the execution of laws and so is obliged to take the regulatory measures after a reasonable delay".
The ecotax set off a storm of protest, especially in Brittany where farmers argued it would make costs of transport prohibitive because of the distance to Paris.
The government climbed down and suspended the move.
The Council of State says that, although a move to repeal the tax is currently making its way through parliament, it has not yet been voted through, so the government is under a legal obligation to enact the previous decision.