Creating a "northern powerhouse" is vital but it should not come at the expense of a "diminished" London, Chancellor George Osborne has said.
He stressed that rebalancing the UK economy was a government ambition but said it should not be achieved by "pulling down" the capital city.
He set out his economic vision for the north west in a speech in Manchester.
Labour said the city had paid "a heavy price" for the government's "failed" economic plan.
The chancellor and the prime minister addressed an audience of business leaders in Manchester, during the start of a two-day visit to the north-west of England.
Mr Osborne said: "Rebalancing our national economy, ensuring that the economic future of the north is as bright, if not brighter, than other parts of the UK, is the ambition we should set ourselves.
"We achieve that not by pulling down our capital city, or diminishing its success. Having one of the greatest global cities on earth, located 200 miles to our south, should be an asset, not a weakness."
The chancellor restated the government's objective of creating a "northern powerhouse of jobs, investment, prosperity and bright futures".
He said closing the economic gap between north and south would bring more than £18bn to the region by 2030.
Mr Osborne and David Cameron also promised to create 100,000 more jobs in the region, and to invest in transport.
Mr Cameron said a strong UK economy depended on no part of the country being "left behind", as he pushed the plan for a so-called northern powerhouse.
Lucy Powell, Labour's general election co-ordinator and Manchester Central MP, said no-one would be fooled by the prime minister's "weasel words", saying living standards had fallen across Manchester since 2010.
Labour has promised to devolve £30bn of spending to the English regions to boost economic growth outside London.
Lib Dem Deputy Prime Minister Nick Clegg has said Manchester, Leeds and Sheffield should become a "Northern golden triangle" to drive economic growth in the north of England.