The family of six-year-old Dáithí MacGabhann have said they are now "taking the fight to London" after a Stormont recall failed to progress stalled organ donation laws.
MLAs returned to the Assembly on Tuesday in a bid to progress the legislation named after the Belfast boy who is waiting for a heart transplant.
But the DUP once again vetoed the election of a Speaker, meaning no further business could be conducted.
Read more: Stormont fails to progress stalled organ donation law - as it happened
A man in the Assembly chamber's public gallery shouted "joke" as MLAs brought their hour-long meeting to a close after failing to elect a Speaker.
It was the sixth time since last May that the DUP has blocked restoring the Assembly as part of its protest against Brexit’s Northern Ireland Protocol.
The opt-out organ donation law, which was named after Dáithí when it was passed last year, has been delayed amid Stormont's collapse because of the need for secondary legislation to implement it.
After Stormont's failure to act, focus will turn to Secretary of State Chris Heaton-Harris and Northern Ireland's MPs to see if the law can be progressed at Westminster.
DUP leader Sir Jeffrey Donaldson plans to introduce an amendment to the government's Executive Formation Bill to facilitate the passing of the regulations.
It will be for the Speaker's Office at Westminster to decide whether to permit the DUP’s planned amendment to the bill, which is aimed at extending the deadline for holding fresh Assembly elections.
But the Northern Ireland Office has said the prospect of this being permitted is "highly unlikely" as the scope of the bill is "very narrow".
Dáithí and his parents Máirtín MacGabhann and Seph Ni Mheallain were at Stormont to watch the ill-fated attempt to restore the institutions.
Máirtín described the outcome as disappointing but unsurprising, and said they had hoped that on Valentine's Day, there may have been a chance of a "fairytale ending" for Dáithí's Law.
He added: "We're not at all surprised but we’ve never lost hope, and even up until that last second we still had a wee bit of hope. But I suppose today gives us the opportunity now to solely focus on Westminster, the Secretary of State, to Jeffrey Donaldson's amendment.
“We'll want to speak with our MPs again, and there was talk of a joint amendment. It's very clear from the floor (of the Assembly) today that all parties are in support and it’s just very disappointing that it couldn’t get over the line today.
“We’re not letting this rest, we're still going to fight, what we’ll probably do now is look at flights for next week.
"Now we are taking the fight to London."
The opt-out system, which aims to increase organ donation rates, would mean adults in Northern Ireland would be presumed to be donors, unless they decide to opt out.
Rival parties had attempted to ramp up the pressure on the DUP to end its boycott of devolution, but the unionist party said the required regulations can be passed at Westminster.
Addressing the Assembly, former DUP First Minister Paul Givan criticised the Northern Ireland secretary's handling of the issue.
He accused Mr Heaton-Harris of delivering different messages in private to the parties compared to his public statements.
He told MLAs: "The Secretary of State is not handling this issue in a way which should be befitting of the office he holds and I appeal to him to work in a manner which is much more constructive and apolitical than the approach that he has taken to date.
"The government have legislated on other issues – same-sex marriage, abortion, the Irish language – and they were able to do that on issues which were much more controversial than an issue like this, which we are all united upon, which all of the parties in this chamber have collectively called upon the Secretary of State to pass at Westminster given that, in the absence of an Assembly and the Executive, it remains the sovereign Parliament to do so."
Sinn Féin vice-president Michelle O’Neill told MLAs the recalled sitting was not about party politics.
She said: "Over 130 people are currently waiting for an organ, 90% of people in the north support organ donation, and every party in this chamber supports this law.
"There is little else to be said. The power is in the gift of each party and every single MLA to save lives by legislating in this place here today.
"Put simply, not to do so is a dereliction of duty, and it’s really disheartening for all those families involved.
"There is an onus on all of us here today to work together and most importantly to give hope to all those families who need us to get this done."
Alliance leader Naomi Long urged the DUP to "do the right thing" and allow a Speaker to be elected.
"That would open the door for this Assembly to pass the regulations and give people who are waiting for transplant surgery a better chance of survival, because for them, this is a life and death issue," she said.
Robin Swann, who was Health Minister when Dáithí's Law was first approved by the Assembly, said the legislation was one of his proudest achievements.
The Ulster Unionist MLA said it was a law that would "not just save lives, but change lives".
Mr Swann said while his preference was for the law to be implemented at Stormont, he urged the government to ensure it went through at Westminster.
SDLP MLA Colin McGrath said the ongoing failure to elect a Speaker was an “absolute embarrassment”.
He said passing the regulations at Westminster was the "second-best option" as it would delay their implementation.
"Everyone that is on that transplant list deserves so much more better than second-best option," he said.
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