Johnson took office on Wednesday pledging to do a new Brexit deal with the European Union but also to ramp up no-deal preparations. He has announced a raft of spending pledges including increasing police numbers by 20,000 to help cut crime.

After a junior interior minister responsible for policing, Kit Malthouse, said Johnson had announced there was to be an emergency budget in the autumn, the prime minister's spokesman said there were no plans for one.

"There are no plans to do so," PM Johnson's spokesman said when asked if Britain would be holding an emergency budget. "Fiscal events, as you know, are announced by the Treasury."

Johnson has not announced an emergency budget. The finance ministry, known as the Treasury, said it was up to Sajid Javid, the finance minister whose official title is Chancellor of the Exchequer, to announce fiscal events.

Typically an autumn budget takes places between late October and early December.

If Johnson is unable to convince the European Union to do a new Brexit deal and lawmakers try to prevent a no-deal exit then there is speculation that he could call an election.

Johnson, who made more than 30 billion pounds ($A53 billion) worth of spending commitments during his leadership campaign, said on Thursday his pledges had been modest "so far".

He has said spending could be financed using the 27 billion pound ($A48 billion) "fiscal headroom", an amount of extra borrowing possible without breaking a budget deficit cap of two per cent of GDP introduced by former finance minister Philip Hammond.

Javid said during his failed bid to lead the Conservative Party that he would prepare for a no-deal Brexit with an emergency budget that would include tax cuts for businesses and individuals.

Before he was appointed to the post, Javid said he wanted to cut tax for the lowest earners but would also consider scrapping the top rate of income tax, if he thought it would inject more dynamism into the economy.

"The prime minister has also announced there's going to be an emergency budget in the autumn which will be designed to stimulate the economy," Malthouse told Sky News.

"What you hope is that a more strongly growing economy will produce greater tax revenue which we can pay for some of the things that we want to do," he said.

Malthouse said a pledge to boost police numbers would cost about 500 million pounds ($A895 million) in the first year.

Malthouse said: "There's certainly headroom in the finances."

"We've gone from a situation at the start of 2010 where we were borrowing one pound in every four that the government spent to now borrowing one pound in every thirty four so we've got a bit of room for manoeuvre."

Australian Associated Press