Jeremy Corbyn believes that there is majority support for unification across both Irish nations, his official spokesman has said ahead of his first visit to Northern Ireland as Labour leader.
The opposition leader is committed to the Good Friday Agreement and any change must happen through "consent", the spokesman added.
Mr Corbyn is due to visit Belfast on Thursday to mark 20 years since the agreement which cemented the peace process.
He will also talk about the issues around Brexit, with the Irish border and customs arrangements currently dividing the Government.
Mr Corbyn is a prominent and long-time supporter of the unification of Ireland and his views have sometimes placed him at odds with official party policy.
Pressed on Mr Corbyn's views on Irish reunification, his official spokesman said: "Over the years he has made his position clear that the majority of those people across the whole island of Ireland wanted to see that outcome, a united Ireland.
"But in the context of the Good Friday Agreement that can only come about through that constitutional process that is laid down in the agreement and Jeremy fully supports that."
Under the terms of the Good Friday Agreement, a united Ireland can only be brought about with the majority consent of people in Northern Ireland and the Republic of Ireland.
In 1984, a decade before the first IRA ceasefire, he met with Sinn Fein leader Gerry Adams in London and, a year later, he opposed the Anglo-Irish Agreement saying it strengthened rather than weakened the border.
Mr Corbyn's spokesman said that during his visit he would seek to "engage with all communities and people across Northern Ireland" on both the peace process and Brexit, as well as "the need for a transformation of the economy in Northern Ireland as in the rest of the UK in a way that works for all communities".