The government is “operating on the assumption” that Britain will leave the European Union without a deal on 31 October, Michael Gove has said.

The new chancellor of the Duchy of Lancaster, who is in charge of no-deal preparations in the Cabinet Office, said there was a “very real prospect”(£) that an agreement would not be struck with Brussels before that deadline.

Writing in the Sunday Times, he said that while the aim was still to leave with a deal, the government needed to prepare for every eventuality. “With a new prime minister, a new government, and a new clarity of mission, we will exit the EU on October 31. No ifs. No buts. No more delay. Brexit is happening,” he said.

“The EU’s leaders have, so far, said they will not change their approach – it’s the unreformed withdrawal agreement, take it or leave it,” he added. “We still hope they will change their minds, but we must operate on the assumption that they will not.”

Gove said the government would do “everything in our power” to secure a good deal for the UK, but said simply presenting Theresa May’s withdrawal agreement to Parliament again would not be enough. “You can’t just reheat the dish that’s been sent back and expect that will make it more palatable,” he wrote.

Planning for no deal was now a “number one priority”, he said, as the paper reported that Boris Johnson had put together a “war cabinet” of six key ministers to deliver Brexit by 31 October “by any means necessary”.

Chancellor Sajid Javid has said there will be “significant extra funding” this week to get Britain “fully ready to leave” the EU on 31 October, with or without a deal.

The additional spending will include financing one of the country’s “biggest ever public information campaigns” to ensure individuals and businesses were ready for a no-deal exit, Javid told the Sunday Telegraph.

“Under my leadership, the Treasury will have new priorities and will play its full role in helping to deliver Brexit,” he said. “In my first day in office as chancellor, 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.”

He added that he planned to fund 500 new Border Force officers and look at new infrastructure around the country’s ports to minimise congestion and ensure goods could flow.

Meanwhile, polls have suggested the Tories were boosted by a “Boris bounce” after the election of their new leader. Since Johnson became prime minister, the Conservatives have gained 10 points to stand at 30%, new polling by Opinium for the Observer shows.

The polling shows the Conservatives benefitting from a surge in support away from The Brexit party.

Despite being pushed into second place, Labour are also up three points to 28% at the expense of the Green party.