Image source, ReutersRishi Sunak bears ultimate responsibility for a Conservative Party mired in "chaos", Ben Houchen has said.
The newly re-elected Tees Valley mayor said "lots of" Tories were "fighting each other like rats in a sack" and the blame "ultimately lies with Rishi".
His comments will come as a blow to the prime minister, still reeling from disastrous local election results.
He had hailed Lord Houchen's victory as a sign the Tories could still win the general election.
The Conservatives lost 474 council seats, the Blackpool South by-election and the West Midlands mayoral race.
Mr Sunak's woes deepened on Wednesday with the surprise defection of Dover MP Natalie Elphicke to Labour in protest at his record on housing and stopping small boat crossings.
Asked if the prime minister was to blame for the Conservatives' problems, Lord Houchen told BBC Radio Tees: "Ultimately it always rests on the shoulders of the leader, all responsibility goes back to the top, it's the same in my job as well.
"Ultimately, you're the one responsible for it. But there are lots of people who are involved in the problems with the Conservative Party. It's a bit of chaos at the minute, right, isn't it?
"There's lots of people fighting with each other in the Conservative Party, there are defections going on, and ultimately the public do not vote for parties who are not united and are not presenting a united front and also aren't talking to the public.
"If they're fighting with each other like rats in a sack instead of saying to the public 'this is what we're going to do for you', that doesn't win elections.
"Obviously, it ultimately lies with Rishi but there are lots of people that need to get their act together, stop messing about and start talking to the public about what they can offer them, rather than just fighting with each other," Lord Houchen added.
Hospital fight
Mr Sunak travelled to Darlington last Friday to celebrate Lord Houchen's re-election, saying it was evidence of "the Conservatives delivering" and "levelling-up in action".
The prime minister said the people of Teesside "knew it was Ben and the Conservatives that delivered for them and they stuck with you at this election. And I know that come the general election, they're going to stick with us too."
Lord Houchen thanked Mr Sunak for his support, saying: "I could not have delivered the things we've delivered in this region without you, prime minister."
Labour said Lord Houchen had run his campaign as a "pseudo-independent" by not branding himself as a Conservative or mentioning Mr Sunak in his campaign literature.
In his BBC interview, the mayor vowed to fight for government funding for a new hospital to replace North Tees hospital in Stockton-on-Teees, an ageing estate he said had a lifespan of "five to seven years, maximum".
"We need a new hospital, that is accepted by everybody. That work has to happen now, it's a priority locally."
Lord Houchen also outlined his plans for a new tram system in parts of Teesside, and said under-21s in the region would now benefit from £1 bus fares.
Asked how he would tackle child poverty, he said he would focus on tackling the cause rather than the symptoms and concentrate on creating high-paid jobs.