David Cameron says the report is not connected to Tory policy
David Cameron has described as "insane" a report by a right-leaning think tank, which suggested some cities in northern England were "beyond revival".
The Tory leader, who is on a visit to target seats in the north, said his party wanted to renew northern cities.
The Policy Exchange report said coastal cities like Sunderland and Liverpool had "lost much of their raison d'etre".
Labour's former deputy leader and Hull East MP John Prescott said the report was "insulting and ignorant".
The report suggested current regeneration policies were failing and mass migration to London, Cambridge and Oxford would stop people becoming "trapped" in poorer areas.
It said the largest coastal cities like Liverpool and Hull had built up for reasons that had since disappeared - like ship building.
"They have lost much of their raison d'etre and it is hard to imagine them prospering at their current sizes," it said.
Policy Exchange, a registered charity, has been described as Mr Cameron's favourite think tank.
Residents criticise the report and defend life in the north
But Mr Cameron, who will be keen to minimise any embarrassment as he tries to gain ground in traditional Labour heartlands, distanced himself from the organisation's findings on Wednesday, saying the report was "insane".
Speaking on a visit to Carlisle, he said: "This report has got nothing to do with the Conservative Party, this is an independent think tank, it has charitable status, I think this report is complete rubbish."
"Conservative Party policy will continue the good work of regenerating cities right across England, including northern cities."
"When I think of urban regeneration and city regeneration I think of things like the Albert Dock in Liverpool, the canal work in Leeds, the SAGE centre in Gateshead.
"I think the idea that cities can't regenerate themselves, they were built for one purpose and can't do another purpose, is just nonsense."
But Labour minister Vera Baird, MP for Redcar, said the report was "exactly the sort of vindictive, anti-northern thinking that led to the widespread industrial decline of swathes of the north under Thatcher".
"Cameron can distance himself from this all he wants but he needs to explain why his friends have no faith in the North," she said.
We should be talking up our city not doing it down
And former deputy PM Mr Prescott said: "To state that northern cities like Bradford, Liverpool and my home town of Hull have no hope of being regenerated in the 21st Century and that people should move to London, Oxford and Cambridge is the most insulting and ignorant policy I've ever heard."
Mr Prescott, whose plan to hand control of regional development budgets to elected assemblies was rejected by voters in the North East in 2004, said the north had experienced a "remarkable renaissance under Labour".
But he said the Conservatives "still see people as economic units to be moved around, not as people with families who want to live and work in the cities they were born in".
The government said it did not agree with the report's conclusions and said since 1997 cities like Liverpool and Newcastle had benefited from thousands of new jobs, lower crime and better living standards.
And the report has angered elected representatives in those areas singled out as being beyond economic recovery.
Louise Ellman, Labour MP for Liverpool Riverside, said the report had "dismissed a huge area of the country as worthless".
"Liverpool is an excellent example of a city regenerating fast - with unemployment halved and increasing inward investment in the year we celebrate being European capital of culture, we should be talking up our city not doing it down."
Meanwhile Warren Bradley, Lib Dem leader of Liverpool city council, said the city - the 2008 European Capital of Culture - was one of the UK's safest, tourism was booming and he said the report had been "widely ridiculed".
Policy Exchange was set up in 2002 by Nicholas Boles, until recently London Mayor Boris Johnson's chief of staff, and Michael Gove, now shadow education secretary, to generate new policy ideas on communities and individual freedom.
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