Sir Keir said Mr Johnson was not being “straight” with the people of Northern Ireland about the consequences of the protocol he agreed with the EU as part of his Brexit deal.
And he said that the PM’s failure to make a positive case for the United Kingdom was putting the four-nation union under “considerable strain”.
Sir Keir was speaking during a three-day visit to Northern Ireland amid fears that rows over Mr Johnson’s deal will escalate tensions around the 12 July marching period.
He warned that distrust was being fuelled by government plans to end Troubles-related prosecutions of military veterans, which he predicted will “unravel” as legislation is brought forward.
A furious row is raging in Northern Ireland over a giant bonfire of wooden pallets erected at Tigers Bay in north Belfast, at an “interface” area between traditionally loyalist and nationalist areas.
DUP leader Sir Jeffrey Donaldson visited the area on Thursday to declare that the pyre should remain, after Stormont’s SDLP infrastructure minister Nichola Mallon insisted the fire must not be allowed to proceed.
Ms Mallon and Sinn Fein communities minister Deirdre Hargey have written to the Police Service of Northern Ireland (PSNI) to warn they will seek judicial review against the decision not to intervene on the bonfire, set to be lit at the weekend as part of traditional “Eleventh Night” events.
They have received agreement from Belfast City Council to remove the pyre, but police have refused to provide contractors with protection for the operation, arguing it would risk disorder.
Deputy first minister Michelle O’Neill said: “Quite frankly any government minister shouldn’t have to take the PSNI to court to do their job.
“I’ve been with residents this week, residents whose homes have been attacked, whose windows have been smashed by masonry being fired at their homes.
“The PSNI should move in to remove the bonfire. Bonfires are not a celebration of culture and they should not be put into an interface area which heightens tension and causes bother.”
Brexit minister David Frost said that the government would condemn any acts of violence, but said that protests on the streets of Northern Ireland were driven by discontent over the implementation of the protocol agreed with Brussels as part of Boris Johnson’s EU withdrawal deal.
Giving evidence to the Northern Ireland Assembly’s Executive Office Committee at Stormont, Lord Frost repeated claims that disruption to supplies of food and medicines were caused by the EU’s over-zealous application of the terms of the deal.
He said the protocol - which forms part of an international treaty signed by Mr Johnson - was “not definitive” and indicated that London remains ready to tear it up by invoking Article 16.
Urged to condemn any threat of violence during marching season, Lord Frost told MLAs: “Obviously we condemn violence and we condemn incitement to violence. That is wrong, and we thinik there should be calm and these problems should be resolved politically.
“But what we are seeing on the streets of Northern Ireland, in terms of protests and so on, seems to me to be a manifestation of quite a lot of discontent about the protocol. Peaceful protest is a perfectly legitimate right in this country. Where people don’t support things, they protest about it.”
Speaking to the BBC’s Good Morning Ulster, Sir Keir called for all sides to “de-escalate the tension” and said that Mr Johnson and Northern Ireland Secretary Brandon Lewis should come to the province to find practical solutions.
The Labour leader said that his discussions with political leaders, community groups and victims of violence in Northern Ireland had been characterised by “a complete lack of trust in the government”.
He said: “Almost everybody in Northern Ireland feels that the prime minister negotiated the protocol, he mis-sold it - misrepresented what its consequences really are - and now he is not taking responsibility for fixing it.
“I’ve come to hear what political leaders have to say and what community groups have to say and they’re saying that very, very loud and clear.
“The prime minister’s role should be as an honest broker. That’s what John Major, Tony Blair and others have done in the past. This prime minister has abandoned that role and I think many people feel betrayed by that.”
Sir Keir said that the UK should forge an agreement with the EU on common veterinary standards, which would resolve problems over the movement of food products between the British mainland and Northern Ireland.
And he rejected Unionist calls for the scrapping of the protocol, insisting that a way had to be found to make the agreement work.
Mr Starmer said that progress in the Northern Irish peace process had “stalled” under Mr Johnson.
“I’m concerned at the number of issues that are swirling around and the difficulties there are at the moment that I think are creating quite a fragile environment,” he said.
“Here we are in what is always pretty difficult week coming up in Northern Ireland. It needs a steady hand. It needs the prime minister or secretary of state here, having discussions with political leaders in the way that I’ve been, trying to find practical solutions.”
The Labour leader said he would not back legislation to end prosecution of veterans and paramilitaries for Troubles-era incidents.
“The idea that the prime minister of the United Kingdom simply says ‘I’ll give up on it now, draw a line, an amnesty for all those involved’ I think is wrong in principle,” said Sir Keir.
“I think it’ll unravel, particularly when his own MPs begin to understand what it is that’s being proposed.”
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