Boris Johnson's new Brexit plan hinges on sign-off from Northern Ireland by next summer - despite the failure to restart power-sharing at Stormont in nearly three years.

Northern Ireland has been without a functioning executive since January 2017 but the prime minister wants to give assembly a vote on proposals by July 2020 - six months before the end of the transition period.

UK officials “absolutely believe” that the devolved assembly could be up and running in nine months to run the rule over Mr Johnson's offer.

But in a sign of the trouble ahead, Sinn Fein warned that the prime minister's DUP allies would never be allowed to wield a veto at Stormont over the Brexit plans.

Mr Johnson submitted proposals for a replacement for the Brexit border backstop to Brussels on Wednesday, with a warning that an inability to reach agreement would be "a failure of statecraft for which we would all be responsible".

His offer proposes an all-Ireland regulatory regime for goods, effectively creating a border down the Irish Sea, and insists Northern Ireland must remain within the UK customs area after the end of a transition period in 2021, meaning customs checks will be required between the Republic and the North.

But the plans hinge on consent from Stormont, which would have a veto before the arrangement comes into force in 2021, and its consent would have to be renewed every four years.

A UK government official said: "Discussions have been ongoing for a number of months now. I think it is vitally important that the people and institution of Northern Ireland are able to have their say over these arrangements.

"That is something that was lacking from the original backstop and that is what the prime minister is looking to address."

Asked what would happen if Stormont had not yet reconvened by July, the official said: "There is a reason why we need to work hard and work hard with the parties to ensure Stormont is up and running by that point."

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Officials were unable to say what trade arrangements would apply in Northern Ireland if Stormont rejected the plans, as the papers say that "arrangements will default to existing rules" if consent is withheld.

"That would have to be a discussion we have closer to the time," said a UK government official.

Ministers could offer extra cash to Northern Ireland to help it prepare for its post-Brexit future, with an official saying the move could have "financial implications".

The Northern Ireland parties reacted with anger, with Sinn Feinn blaming the deadlock over Stormont on the DUP and the Tories.

Michelle O'Neill, Sinn Fein deputy leader, said: "There can be no situation whatsoever when the DUP are going to be afforded a veto, a lock, a blocking mechanism of any form in which to thwart any progress that could potentially be made on Brexit between both the EU and the British government.

"It is just a nonsense, an illogical statement to make and to keep making it over and over again."

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Robin Swann, leader of the Ulster Unionists, said the plans would leave Northern Ireland "locked into continual political debates" on Brexit and the relationship with the UK.

"They would set the theme of every Assembly and Westminster election. It plunges Northern Ireland into a referendum in the Assembly chamber every four years with high stakes consequences for our people.

"It will keep our businesses and agri-food sector in a perpetual cycle of uncertainty."

However DUP urged all parties to approach discussions with a "positive mindset within a spirit" of wanting a new deal.

The statement said: "Further work remains to be completed between the UK and the European Union but we would encourage all concerned to approach these discussions in a positive mindset within a spirit of wanting to secure a negotiated withdrawal agreement that can allow everyone to focus on future relationships.

"For our part that is the manner in which we will approach these issues over the coming period."