David Cameron has convened the Cabinet to discuss the newly-struck EU renegotiation deal as he prepares to announce the date of the in/out referendum.
The meeting will effectively fire the starting gun on the campaign, expected to culminate in a vote on June 23.
Among ministers around the table are a number who favour Brexit - including Iain Duncan Smith and Chris Grayling - who will be free afterwards to publicly back the "Leave" camp.
Mr Cameron has said he is "disappointed but not surprised" that they are set to be joined by his long-time political ally - but consistent advocate of withdrawal - Justice Secretary Michael Gove.
After lengthy talks with his fellow EU leaders in Brussels finally produced a package of changes, the Prime Minister declared late last night that he would campaign "with all my heart and soul" for the UK to stay in the 28-nation bloc.
He said the changes - including curbs on EU workers' benefits, protections for non-euro nations and an opt-out from "ever closer union" - cemented Britain's "special status" despite a series of compromises.
Chancellor George Osborne said the package "addressed the major concerns that many people have had" and warned that quitting the EU was "a huge leap in the dark with the risks that entails for our country, for its economy and for our security".
But Eurosceptics - including many within Mr Cameron's Conservative Party - dismissed the package as meaningless and said only withdrawal could restore sufficient powers to the country from Brussels.
And Labour leader Jeremy Corbyn, while confirming the Opposition would campaign for an "in" vote, dismissed the re-negotiation as a "missed opportunity".
"We will be campaigning to keep Britain in Europe in the coming referendum, regardless of David Cameron's tinkering, because it brings investment, jobs and protection for British workers and consumers," he said.
Ukip leader Nigel Farage dismissed the "truly pathetic deal" and urged voters to seize the "golden opportunity" to show that Britain would be better off being fully independent of Brussels.
The new deal will allow host nations to cut migrants' child benefit payments for children living overseas to the rate paid in their home countries - usually far lower than those received by UK parents.
A compromise will allow existing claimants to carry on receiving child benefit in full for offspring living overseas until 2020, and all member states will then be able to pay them at the rate of their home country.
It falls well short of the outright ban on sending child benefit abroad initially demanded by Mr Cameron, and marks a compromise with eastern European states who had insisted that existing claimants should continue to receive the full payment until their sons and daughters reach adulthood.
An "emergency brake" on in-work welfare payments for future migrant workers will be made available for seven years - with no option for extensions - in cases where member states are facing excessive strain from new arrivals.
The seven-year period is shorter than the 13 years put forward by Mr Cameron in negotiations, but considerably longer than eastern European nations had argued for.
The new deal also says EU treaties will be amended to state explicitly that references to the requirement to seek ever-closer union "do not apply to the United Kingdom".
Mr Osborne said he respected Mr Gove's long-held view but believed "the majority of people want to be in a reformed EU" and most ministers would stick to the Government line.
He confirmed that proposed domestic legislation to assert the sovereignty of the UK Parliament would be set out shortly - with the Prime Minister expected to give details when he appears on BBC1's Andrew Marr Show on Sunday.
The issue is important to London Mayor Boris Johnson, an influential voice within the Conservative Party who is yet to declare on which side he will campaign.