Jeremy Corbyn has insisted Labour's pro-EU message is getting through to voters.
It comes after the leader of one of the biggest unions said he needed to do more to convince Labour voters of the case for a remain vote.
Tim Roache, general secretary of the GMB union, said Labour had been "silent for too long" on key issues and needed to "bolder and braver" on key referendum issues such as immigration.
He told the BBC that he was concerned Labour supporters "would stay at home" on polling day on June 23.
But Mr Corbyn insisted the party was getting its message out "as loudly as we can".
Asked why a recent poll suggested Labour voters do not actually know which side the party is on in the referendum debate, he suggested it was "partly down to the media" - prompting huge cheers from activists.
He told ITV News' Chris Ship there were "no no-go areas" in the party's campaign.
"I don't think anyone is going to be in any doubt as to what our views are come the vote on June 23."
In his speech, the Labour leader also:
- said Labour would push for reforms in the EU including to strengthen workers' rights
- called on the prime minister to reject proposals to ease trade between the US and the EU - Transatlantic Trade and Investment Partnership (TTIP)
- attacked George Osborne over his Brexit recession warnings
Earlier, shadow cabinet member Owen Smith and senior backbencher Mary Creagh also backed Mr Roache's warning.
Mr Smith, the shadow work and pensions secretary, said he will be "urging Jeremy to redouble his efforts" to present a "strong and distinct Labour case" for the 'Remain' campaign.
Mr Corbyn has been accused of taking a low-key role in the 'Remain' campaign to date.
A long-time critic of the EU, the opposition leader voted against membership of the European Economic Community in 1975, and has refused to share a platform with the Tories - despite other high-profile Labour figures, including newly-elected London Mayor Sadiq Khan, joining forces.