The Vote Leave campaign group said the £9.3m leaflet drop puts the legitimacy of referendum at risk while pro-Brexit Boris Johnson said it was 'crazy' to use tax payers money on something intended to 'scare people'.
While the officially designated campaign groups on each side of the June 23 referendum will each get a chance to deliver leaflets to voters, Vote Leave say the Government's pro-EU stance has effectively given the the remain campaign a further boost.
"David Cameron is discrediting the referendum process with his taxpayer-funded dodgy dossier," said Liz Bilney, CEO of the Leave.EU campaign.
"How can it be right that the Remain campaign is allowed two national mail shots and the Leave campaign only one? This is supposed to be an equal contest.
"The Prime Minister, who knows he has a weak case and repeatedly resorts to scaremongering, is trying to load his gloves. He is ruining Brand Cameron, ruining the reputation of British democracy and putting the legitimacy of the final result at great risk."
The leaflet drop cost the tax-payer £458,500 to create, and a further £5,947,436 has been spent on printing and delivering it to over 27 million homes across the UK in two waves.
Another £2,894,064 will go on producing the website and promoting it via social media and other online platforms - a total cost of around 34p per household.
Downing Street said the move was a response to Government-commissioned polling which showed 85% of the public wanted it to supply more information to help them make an informed choice on June 23.
Environment Secretary Liz Truss said it was "crucial that the public have clear and accessible information".
Speaking to ITV, London Mayor Boris Johnson, who is campaigning to leave the EU, said: "If this thing was genuinely going to help people understand what is going on in the debate, there might be a case for it but given that I think it's very likely that it's going to be very biased and hysterical and warning unnecessarily about the risks of leaving the EU, I think it's a complete waste of money.
"It's crazy to use quite so much tax payers money on stuff that is basically intended to scare people and to stampede people in one direction.
"If you are going to use tax payers money you should allow people to put the other side of the case as well."
However Tory MP Peter Bone, co-founder of the Grassroots Out campaign, said it was "immoral, undemocratic and against what the Government has promised".
He said: "Many recent polls have shown that the majority of the UK public are actually in favour of leaving the EU so to spend their money on a pro-EU propaganda exercise is an inexcusable waste.
"The Prime Minister promised Parliament that no taxpayers money would be spent promoting remain or leave. If this is not reversed it will seriously damage the Prime Minister's reputation."