Labour supporters have "most to gain" from keeping Britain in the EU, Gordon Brown will say as he spearheads a referendum charm offensive.
With just 10 days until the country decides whether to terminate its 43-year stay in the bloc, Opposition figures are being thrust centre-stage in a late bid to seize the initiative.
Pro-EU leaders are concerned that the potentially pivotal group of voters in a tight tussle are being pushed towards Brexit by the campaign's focus on Tory rivalries and immigration.
But Mr Brown will argue their interests are best served by keeping the UK at the table in Brussels to secure reforms to boost jobs, cut bills, enhance workers' rights and tackle tax havens.
Fears over the impact of foreign workers on local communities would be better addressed by insisting on more money to boost stretched public services than ending free movement rights, he will argue.
And shadow foreign secretary Hilary Benn will make the "patriotic" case for a Remain vote - arguing Britain's modern strength has been drawn from its work as a leader within international alliances.
Only a handful of Labour MPs have broken ranks to back the Leave camp - but leader Jeremy Corbyn has faced claims of giving only lukewarm support to the party's official position.
Backbencher Gisela Stuart - who chairs Vote Leave - said "no amount of hastily cobbled together relaunches" would win back Labour backers who have "seen through the spin of the Government".
Mr Brown - whose late intervention in the Scottish independence referendum was considered an influential factor by some observers - will set out his "positive" message in a speech.
The UK would be "in pole position" to champion change when it takes the rotating presidency of the EU in 2017, he will say.
"From now until 10pm on June 23 we will not rest and I will not stop explaining why nine million Labour voters have most to gain from remaining in the EU," he will say.
"Today I am setting out a positive agenda for Labour voters - reasons why Labour voters should vote Remain and the patriotic case for remaining in Europe.
"But we shouldn't just be a member of the European Union. We must be the leader of the European Union."
Mr Brown will argue that reforms to digital, energy and service sectors could create an estimated 500,000 new jobs in Britain over 10 years - and that a pooled energy strategy could cut domestic bills.
The EU's "collective clout" could force action against tax dodgers and secure better protections for workers on zero-hour contracts, he argues.
He is also proposing a European solidarity fund to help communities in which healthcare, schools and public services are under pressure because of sharp population changes.
And a fight by Britain for a large slice of up to 16% of a 315 billion euro (£242 billion) infrastructure initiative to help industries such as steel hit by closures and restructuring.
Ms Stuart said: "We had an opportunity to get positive changes out of the EU but David Cameron blew it with his failed renegotiation.
"Labour voters have seen through the spin of the Government which is why they are rejecting the In campaign and no amount of hastily cobbled together relaunches will change that."
Mr Benn will use a speech to say the Leave camp "sound as if they mourn for the bygone age in which Britain gained influence through military strength and Empire...
"In the second half of the 20th century, we came to realise that it was far better and far more effective to be a global power that achieved its goals through co-operation rather than conquest."
"We have now reached the defining moment in this referendum. The Brexit train is threatening to pull away from the station with Nigel Farage, Boris Johnson and Michael Gove in charge, taking us down a track that can only lead to disaster.
"It would make us poorer, less powerful and it would hurt the next generation as they try to prosper in tomorrow's world. That's why it is so important that we make the patriotic case for remaining in the EU and keeping Britain great."
Shadow chancellor John McDonnell MP said: "Labour wants to better use the EU's levers of power to help British families get more control against the growing economic forces of globalisation."
Some 20 of the party's MPs have signed a letter calling for Labour voices to be given more media airtime amid complaints coverage has centred on Conservative-led "fear" messages.