Jeremy Corbyn will set out Labour’s vision for Brexit when he meets the EU’s chief negotiator in Brussels.
Mr Corbyn will tell Michel Barnier he is “ready to take up the responsibility for Brexit negotiations” if there is a change in government.
Mr Barnier will also hold separate meetings with the first ministers of Wales and Scotland, Carwyn Jones and Nicola Sturgeon.
But he has stressed he will only negotiate with the UK government.
The Conservatives said Mr Corbyn would “surrender”, rather than negotiate, with the EU if he was in charge.
A second formal round of Brexit negotiations is due to begin on Monday, and Mr Barnier has warned significant issues remain between the UK and the EU on one of the first issues to be tackled, citizens’ rights.
Labour says it would unilaterally guarantee EU nationals’ rights and “extend the hand of partnership and friendship” to the rest of the bloc.
“In contrast to the Conservatives’ megaphone diplomacy, we will conduct relations with our European neighbours respectfully and in the spirit of friendship,” Mr Corbyn said.
“Our strong links with our European sister parties gives Labour an advantage in reaching an outcome that works for both sides.”
His visit comes after Foreign Secretary Boris Johnson said the EU could “go whistle” if it wanted an “extortionate” divorce bill from the UK, prompting Mr Barnier to respond: “I’m not hearing any whistling, just the clock ticking.”
Mr Jones said he would use his meeting to push for “full and unfettered access” to the EU single market and to avoid no deal being reached.
The Welsh first minister said he hoped to demonstrate to Mr Barnier that there were “parts of the UK that are prepared to engage constructively with the EU-27, rather than indulge in playing to the gallery”.
Ms Sturgeon’s spokesman said the meeting was “an opportunity to brief Mr Barnier on Scotland’s priorities and seek to enhance our understanding of the current EU position as Brexit negotiations continue”.
He added: “Our priority is protecting Scotland’s vital interests, and building consensus against an extreme Brexit outside the single market, which would be potentially disastrous for jobs, investment and living standards.
“This is not about holding separate Scottish negotiations – we have always accepted that the EU will only negotiate with the UK, which is why we will continue to work hard to influence the UK position.”
The Conservatives dismissed Mr Corbyn’s trip to Brussels, saying Labour was “hopelessly divided” on key issues like immigration.
“Jeremy Corbyn wouldn’t negotiate in Brexit talks, he would surrender. He has made clear Labour would accept any deal on offer- even if it was designed to punish Britain,” Tory MP James Cleverly said.
Brexit: Jeremy Corbyn to meet EU negotiator Michel Barnier}