In a joint letter, northern leaders have told Mrs May the North has a bigger population and economy than Scotland and Wales after she stressed she wanted the whole of the UK to be involved in the Brexit discussions.
The letter is signed by the leaders of combined authorities stretching from Liverpool to Yorkshire include West Yorkshire Combined Authority chairman Peter Box and Sheffield City Region Combined Authority chairman Sir Steve Houghton.
It says: “Your efforts to meet leaders outside London is commendable. However, the North of England has long had concerns that we are being ignored, caught between an economically and politically powerful London and an increasingly politically important Scotland.
“As we negotiate our exit from the European Union, you have made clear that you believe in having an approach and negotiation objectives that include the whole of the United Kingdom. On that point we wholeheartedly agree.
“It is absolutely vital that the voice of the North of England is heard loud and clear during the Brexit negotiations.”
Ed Cox, director of the IPPR North thinktank, said: “The North of England has a population of 15 million and an economy worth £300 billion - twice that of Scotland - and so it is right that Northern leaders should demand a place at the Brexit negotiating table and a North First approach to investment.
“The city leaders should be mindful though that it was outside the big cities that the strength of the Leave vote was strongest and for the Northern Powerhouse initiative to prosper, greater thought must be given to how the North speaks with a more inclusive voice and develops a more joined-up economic plan.”
Mrs May, who visited Scotland on Friday, will visit Wales today to hold talks with First Minister Carwyn Jones on a range of subjects including the future of the steel industry and the Tata Steel plant at Port Talbot.
Speaking ahead of the visit, Mrs May said: “I want to say to the people of Wales that the Government I lead will always be on your side - and that means I will stand up for you and your family against injustice and inequality.”