Research published by think tank IPPR North says small and medium cities contribute £82bn a year to the UK economy, and says their growth rates match that of the big cities.
Leaders had already criticised the powerhouse vision for leaving out Preston and Lancashire, and say the latest research proves the area must be included in the plans.
David Borrow, deputy leader of Lancashire County Council, said: “There’s a lot of people involved with big projects in the north of England who come from those areas.
“We need to make sure those areas such as Lancashire are well connected with the Merseyside, Greater Manchester, West Yorkshire corridor, rather than all the big infrastructure works finishing at the borders and not connecting with Lancashire. “It makes it easier as part of a combined authority to play a bigger role, when Lancashire speaks with one voice.”
Ed Cox, Director of IPPR North, said: “The latest academic research is asking some big questions about the big city story and it is about time policy-makers recognised this. In the same way small and medium businesses are now seen as vital to the British economy and the success of our big companies, we need to refocus policy on the North’s small and medium towns and cities, and not just the big cities.”