The British government is working on the assumption that the EU will not renegotiate its Brexit deal and is ramping up preparations to leave the bloc on October 31 without an agreement, senior ministers said on Sunday.

Boris Johnson, who took over as British prime minister on Wednesday with a promise to deliver Brexit by the end of October "no ifs or buts," plans to seek a new exit deal with the EU. The EU has said that the deal cannot be reopened.

Leading Brexit supporter Michael Gove, who Johnson has put in charge of "no deal" preparations, wrote in the Sunday Times newspaper that the government would undertake "intensive efforts" to secure a better deal from the EU.

"We still hope they will change their minds, but we must operate on the assumption that they will not ... No deal is now a very real prospect and we must make sure that we are ready," Gove wrote.

"Planning for no deal is now this government's No.1 priority," he said, adding "every penny needed" for no deal preparations would be made available.

Gove said the government would be launching "one of the biggest peacetime public information campaigns this country has seen" to get people and businesses ready for a "no deal" exit.

The Sunday Times reported that Dominic Cummings, the mastermind behind the 2016 referendum campaign to leave the EU and now a senior aide to Johnson, told a meeting of the prime minister's advisers that he had been tasked with delivering Brexit "by any means necessary."

Johnson has set up a "war cabinet" of six senior ministers to make decisions on Brexit and is preparing for a no-deal emergency budget in the week of October 7, the newspaper added.

Writing in the Sunday Telegraph, new finance minister Sajid Javid said: "In my first day in office ... I tasked officials to urgently identify where more money needs to be invested to get Britain fully ready to leave on October 31 - deal or no deal. And next week I will be announcing significant extra funding to do just that."