Ministers could be hauled to Parliament to defend spending taxpayers' cash on a pro-EU leaflet after a petition opposing it passed the 100,000 threshold.
The decision to send a Government-produced leaflet to every household in the country setting out the case for voting to remain in the EU in June's referendum - at a cost of more than £9 million - provoked a blistering row between the in and out camps.
Justice Secretary Michael Gove attacked the idea of taxpayers funding "one-sided propaganda" rather than the NHS after David Cameron insisted it was "money well spent".
Downing Street said the move was a response to polling which showed 85% of the public wanted more information from the Government to help them make an informed choice on June 23.
But a petition calling for the leaflet campaign to be halted soared past the 100,000 mark, meaning it has to be considered for debate in Parliament as anger from Leave supporters continued.
The petition text, submitted by Get Britain Out director Jayne Adye, says: "We, the petitioners, demand the Government STOPS spending our money on biased campaigning to keep Britain inside the European Union.
"The Great British Public have waited since 1975 for a vote on our relationship with Brussels. No taxpayers' money should be spent on campaign literature to keep Britain inside the EU."
Prime Minister Mr Cameron earlier said he would "make no apologies" for throwing the full weight of the Government behind one side of the argument, declaring: "It is not, in my view, just legal - I think is it necessary and right.
"I don't want anyone to go to the polls not knowing what the Government thinks, and I think that is money well spent."
But Mr Gove - one of four Cabinet ministers campaigning for the "leave" vote on June 23 - said it was "wrong" and called for a fair campaign in which both sides are heard.
He told the BBC: "I just think it is wrong that at a time of austerity, £9 million of taxpayers' money is being spent on a one-sided piece of propaganda.
"That money should be being spent on the NHS and the people's priorities, not on propaganda."
The cost of the promotional push is greater than the £7 million each the formal Leave and Remain camps will be allowed to spend by law in total during the last 10 weeks of the campaign, Vote Leave said.
Ukip leader Nigel Farage questioned whether the referendum was now still "free and fair".
The Electoral Commission watchdog had campaigned for a ban on such Government activity during the formal "regulated period" of the campaign, which begins on April 15.
The leaflet is due to start hitting doormats in England next week, but not until after May 5 elections in the rest of the UK.
Although ministers backed down from a significant lifting of the pre-vote "purdah" period during which government activity is restricted, legal restrictions still apply only for the final 28 days.
The pro-Brexit Grassroots Out (GO) group has written to the Commission, questioning whether the leaflet was legitimate "given that the Government has not registered as a campaigner".
GO suggested the cost should be included within the spending limit of the Remain camp during the formal campaign period.
A Commission spokeswoman said: "After the referendum on Scottish independence, the Electoral Commission recommended that governments should conduct no taxpayer-funded advertising activity during the regulated period.
"However, Parliament decided not to put any legal restrictions on Government activity until 28 days before the poll, May 27. These are the same rules that were in place for other recent referendums."
She said: "The Electoral Commission is responsible for regulating the rules on spending in the run-up to the EU referendum.
"The rules on spending apply during the regulated period which starts on April 15 and ends on polling day, June 23.
"The rules exclude spending that is met out of public funds, which includes spending by the Government on the Government information booklet."