A legal challenge against Greece's critical bailout referendum has been rejected, clearing the way for Sunday's vote to go ahead.

The Council of State, the country's highest administrative court, threw out a mation brought by two private citizens asking the court to rule the referendum illegal.

Spyridon Nicolaou, one of the two filing the motion, had claimed the would be invalid "because it expressly violates the constitution, which stipulates that a referendum cannot take place on economic matters".

"But it's also invalid because it doesn't incorporate the text of the documents on which the Greek people are called on to decide. Would anyone from Evros (in far north-eastern Greece) know the specific documents?" he added.

A separate group filed a counter-motion supporting the referendum's legality. One backer, Dimitris Belantis, said the Council of State has no jurisdiction over the matter and the constitution stipulates popular votes can be held on "crucial national matters".

Simultaneous rallies are being held in Athens supporting Yes and No answers to a murky question in what an opinion poll suggests could be a very close vote.

Prime minister Alexis Tsipras called the referendum last Friday, asking Greeks to decide whether to accept creditors' proposals for more austerity in exchange for more loans - even though those proposals are no longer on the table.

He said a No vote would put him in a stronger position to seek a better deal for Greece within the 19-nation eurozone to reduce its 320 billion euro (£227 billion) national debt and make payments more sustainable.

In a televised address to the nation, Mr Tsipras urged Greeks to vote "no to ultimatums, divisions and fear". He emphasised that Sunday's referendum is not a vote on whether Greece will remain in the euro.

But opposition parties, and many European officials, say a No vote would drive Greece out of the euro and into an even more impoverished future.

Finance minister Yanis Varoufakis said an agreement with the country's creditors "is more or less done" and that the only issue left is debt relief.

But the head of the eurozone finance ministers' group, Jeroen Dijsselbloem, rejected those comments, saying negotiations are not still going on.

"There are no new proposals from our side and, whatever happens, the future for Greece will be extremely tough," Mr Dijsselbloem said. "To get Greece back on track and the economy out of the slump, tough decisions will have to be taken and every politician that says that won't be the case following a No vote is deceiving his population."

A poll conducted on Tuesday and Wednesday and published in To Ethnos newspaper today showed the two sides in a dead heat. It also showed an overwhelming majority - 74per cent - want the country to remain in the euro, compared with 15per cent who want a national currency.

Of the 1,000 respondents to the nationwide survey by the ALCO polling firm, 41.5per cent will vote Yes and 40.2per cent No, well within the margin of error of 3.1 points. Another 10.9per cent were undecided and the rest said they would abstain or leave their ballots blank.

Both sides were trying to sway the undecided in rallies this evening, to be held 900 yards apart in central Athens. Mr Tsipras is to speak at the No rally in the capital's main Syntagma Square outside Parliament, while Yes supporters will gather at the nearby Panathenian Stadium, where the first modern Olympics were held in 1896.

Meanwhile, Greek's banks are unlikely to reopen on Tuesday, whether there is a Yes or No vote. There is in fact a bank holiday on Monday, a move announced by Mr Tsipras last week. But reports suggest they are down to their last 500m euros (£350m).