Sinn Féin must abandon its long-held refusal to sit in the House of Commons because the future of the island of Ireland for generations could be settled there in the coming years, former SDLP leader Colum Eastwood has said.

A decision on a unity referendum in Northern Ireland will be decided by the NI Secretary of State and by MPs in Westminster, he said. “[It] has become absolutely clear that this is a decision that’s going to be taken in London, whether we like it or not.”

He added: “The idea that we would leave eight or nine MPs sitting in Ireland after the next election with a decision about when a Border poll is going to come, or what it’s going to look like, seems to me like the biggest strategic mistake that Irish nationalism could make.”

Sinn Féin’s refusal to allow its MPs elected to Westminster sit in the House of Commons is “a terrible dereliction of duty”, Eastwood told the annual SDLP conference in Belfast.

Meanwhile, Northern nationalists who want to see a united Ireland “will no longer be told” and the Irish Government “must get the finger out” and start to prepare for it, Eastwood said.

Paying tribute to Taoiseach Micheál Martin for the multibillion-euro Shared Island Fund, which is making real change, Eastwood, however, went on: “I think people in the North want Micheál and the Irish Government to catch up where we are at.”

The rupture that was created by the Brexit referendum a decade ago “hasn’t been felt in the South in the same way that it was in the North,” he told The Irish Times. “It has utterly changed the conversation about the future here.

“We want the Irish Government to catch up and begin planning for a united Ireland. And that’s just where people are at,” said Eastwood, who cautioned against over-interpretations of Martin’s call for talks on agreed rules for a border poll.

A “sensible” British government would never call a unity referendum in Northern Ireland without the agreement of Dublin, “unless it was Reform, or something like that”, said the Derry-based politician.

“The reality is that the two governments are going to have to decide when this thing happens, and how it happens. I think, though, our side of the argument is too distracted at times by issues of date, criteria, all those things.

“I just want to say to people, the campaign has started. Act accordingly. Begin to convince people that this is a good idea. There’s going to be a referendum. That’s going to happen,” he went on.

Nationalists have been “told for 30 years” that they had to subdue their unity ambitions to let all Northern Ireland’s political parties work together and let the Stormont institutions settle down, he told the party’s conference in Belfast.

Insisting that Dublin must start planning for unity, he said the SDLP has made clear for half a century that unity “cannot be left on the back burner” or put to one side until some ill-defined description of reconciliation has been achieved.

“We will not be told any longer that some ill-defined notion of reconciliation has to stop us even considering the idea of Irish unity,” he told the conference, to loud applause from delegates.

Urging party supporters not to get fixated on a date for a unity referendum, he said: “Let’s not get distracted about dates for Border polls, or criteria or anything else. I’m genuinely the most confident I have ever been in the prospects for the future.

“There is going to be a Border poll. We are going to win a Border poll and this country is going to be united. If you believe in that, don’t be distracted by all of the noise.”