The 14-page document is being sent to 27m homes and is aimed at stating the case for the government’s pro-EU position.
It has caused controversy because the campaigning period for the EU referendum has not yet begun.
Once the official campaigning period kicks off the designated Leave and Remain campaigns will be limited to spending £7m on advertising and promotion.
This budget includes £600,000 of public grants that have been set aside for free mailshots and TV broadcasts.
UKIP leader Nigel Farage has argued the government’s £9m mail-out is a "scam" and told the BBC that it supports his view that the referendum is "defined by the battle of the people against the political elite".
However, the prime minister has tweeted that "key facts" outlined in the document explain why the government believes "we are stronger, safer and better off in the EU".
Royal Mail will begin delivering the leaflets, which are entitled Why the Government Believes That Voting to Remain in the EU is the Best Decision for the UK , to households in England next week and in Scotland, Wales and Northern Ireland from 5 May.
A digital version will be promoted on social media and the government has even set up a standalone website setting out its case.
Katie Ghose, of the Electoral Reform Society, told the BBC that "rushing" to spend £9.3m on the mail drop before stricter rules kick in is at risk of making the government appear as if it is "playing fast and loose with the situation".