An all-Ireland forum on Irish unity should be convened without delay, Mary Lou McDonald has said.
Momentum is building around the constitutional shift and there should be political, institutional and legislative arrangements in place to manage that, the Sinn Fein president added.
On Friday, Irish premier Leo Varadkar said moderate unionists and nationalists could be spurred to consider a united Ireland if there is a no-deal Brexit but it would be provocative for his Government to begin making preparations now.
Ms McDonald said: “The Taoiseach must make clear that in the event of no-deal Brexit that Irish unity is the solution to averting a hard border on our island.
“A referendum on unity is expressly provided for in the Good Friday Agreement, an agreement that was endorsed by the people of this island, north and south, overwhelmingly over 20 years ago.
“There is nothing provocative about wanting a united Ireland and the Government has a responsibility to lead from the front.”
On Friday, Mr Varadkar said those uncomfortable with a “nationalistic” Britain which is potentially considering reintroducing the death penalty could join forces to support Irish unity and continued membership of the EU.
He said it would be provocative for the Irish Government to take steps towards engineering a united Ireland now and the country simply wants to maintain the status quo.
Ms McDonald said: “There is a momentum building for Irish unity and we need political, institutional and legislative arrangements in place to manage that.
“That preparation must include the convening of an all-Ireland forum on Irish unity without delay.
“With the threat of a crash Brexit now a live prospect, it is irresponsible not to prepare for constitutional change.”
Ireland has urged the UK to ratify the draft Withdrawal Agreement with the EU, which led to Theresa May’s fall from power.
It supports the backstop insurance policy intended to prevent a hard Irish border and keeping the UK’s trade regulations aligned with the EU’s.
Unionists fear that would threaten the integrity of Northern Ireland’s place in the UK, while Brexiteers believe it could prevent the UK from striking trade deals with other countries.