He charged Brexit had been treated under his predecessor as "an impending adverse weather event".

British Prime Minister Boris Johnson on Saturday said Brexit was a "massive economic opportunity" but had been treated under his predecessor Theresa May as "an impending adverse weather event".

In a speech in Manchester where he pledged new investment in Leave-voting areas, Johnson promised to step up negotiations on post-Brexit trade deals and set up free ports to boost the economy.

"When people voted to leave the European Union, they were not just voting against Brussels, they were voting against London too," he said.

Johnson promises to give more powers to local communities, as well as boost broadband and transport infrastructure in a speech focused on domestic issues.

"Taking back control doesn't just apply to Westminster regaining sovereignty from the EU, it means our cities and counties and towns becoming more self governing," he said.

"Leaving the EU is a massive economic opportunity to do things we've not been allowed to do for decades," he said.

Asked about the prospect of Brexit negotiations, Johnson said he was willing to engage with EU partners but only if the backstop clause was removed from the current divorce agreement struck by May.

The backstop seeks to ensure a free-flowing post-Brexit border between British Northern Ireland and the Republic of Ireland, an EU member state, in all eventualities.

"The approach of the UK government is not going to be disengaged or aloof or waiting for them to come to us, we are going to try to solve this problem," he said.

"We can't do it as long as that anti-democratic backstop, that backstop that seeks to divide our country, divide the UK, remains in place. We need to get it out and then we can make progress."