The two sides in the EU referendum campaign are wheeling out their big guns, as Boris Johnson launches a Vote Leave battlebus tour and Gordon Brown issues a plea for Britain to "lead in Europe, not leave it".
The former mayor of London is set to join Vote Leave chairwoman and Labour MP Gisela Stuart in Truro, Cornwall, for the inaugural journey of a bus which will criss-cross the country over the coming weeks to take the Brexit message to all corners of the UK before the June 23 referendum.
Former prime minister Mr Brown will use a high-profile speech in London to make what he terms the "patriotic" case for Britain to remain a member of the 28-nation bloc.
Liberal Democrat leader Tim Farron will echo David Cameron's warning of the threat posed by Brexit to peace and security, arguing that a Leave vote would risk a return to the "mutual hostility" of a century ago, when Europe was convulsed by the First World War.
Mr Farron, who has criticised the "half-hearted" support offered to the Remain camp by Labour leader Jeremy Corbyn, will use a speech in London to repeat a call for progressive parties to set aside "tribal loyalties" and join him at a mass rally for continued UK membership of the EU.
Writing in The Guardian, Mr Brown said the referendum debate had so far pitched the Remain camp's warnings of economic instability after Brexit against the Leave side's "appeal to the heart" with a vision of the Britain of 1940 "standing alone" as "wave after crushing wave of globalisation" threatens the country.
He insisted that supporters of EU membership should offer a "positive-sum" vision showing how "the right balance between autonomy and co-operation can be struck without putting our national identity at risk".
Britain's traditions and history are "outward-looking and engaged with the world" rather than "insulated and isolated", and its best interests are to balance "the national autonomy we desire with the continental co-operation we require", he said.
Mr Brown played down fears of loss of sovereignty to Brussels, insisting that "the future lies not in a United States of Europe but a United Europe of States".
Recent years had seen national governments of the 28 EU states taking over decision-making power from "a once overbearing European Commission", as the bloc moved away from the earlier fashion for harmonising laws and practices and towards mutual recognition of each country's standards and traditions.
Britain does not need to sacrifice its political and social culture or its national autonomy to benefit from a single market which will be "the biggest British job creator of the next decade", he said.
The former PM said cross-border co-operation and intelligence-sharing between EU states is vital to tackle terrorism, people trafficking and illegal migration, while all of Europe would be "at risk from Russian aggression, Middle Eastern terrorism and African instability" without a common EU security policy.
Only a united Europe - and not Nato or any single country acting alone - could deliver the necessary combination of diplomacy, aid and economic support, he said.
"The June vote should be a salute to Britain's irrepressible spirit, a tribute to our tradition of looking outwards and a progressive, agenda-setting moment that shows European co-operation is the best way to secure more jobs: the one way to curb tax havens, the main way to tackle illegal immigration and terrorism on our borders, and a progressive way to tackle climate change and set minimum standards at work," wrote Mr Brown, who sets out his argument in greater detail in a book entitled Britain: Leading not Leaving.
"A positive-sum moment can be borne out of what can sometimes seem like a zero-sum referendum, as we demonstrate that we best honour our outward-looking internationalism by leading in Europe, not leaving it."