Sir Keir Starmer said the landslide victory in the General Election has given Labour “a clear mandate to govern for all four corners of the United Kingdom”.

Facing his first media grilling as Prime Minister on his first full day in Downing Street, he said he was “restless for change” but warned that changing the country “is not an overnight exercise”.

After sweeping to a historic victory at the polls, Sir Keir said his party had received “a mandate to do politics differently”.

“This will be a politics and a Government that is about delivery, is about service. Self-interest is yesterday’s politics.”

Sir Keir also said: “We clearly on Thursday got a mandate from all four nations.

“For the first time in 20-plus years, we have a majority in England, in Scotland and in Wales.

“And that is a clear mandate to govern for all four corners of the United Kingdom.”

He set out plans to travel on Sunday to Scotland, Northern Ireland and Wales, before returning to England, during which time he would meet First Ministers and “establish a way of working across the United Kingdom that will be different and better to the way of working that we’ve had in recent years and to recognise the contributions of all four nations”.

Answering questions from journalists, Sir Keir said: “I am restless for change and I think and hope that what you’ve already seen demonstrates that.”

He pointed to the appointments of Sir Patrick Vallance and business chief James Timpson as evidence of his party’s commitment to change.

“The thing that’s changed already is the mindset of the Government,” the Prime Minister said when asked if he could offer one concrete promise to voters about delivery in the first 100 days.

“We have been planning for months to hit the ground running,” Sir Keir said, adding: “But look, it is not an overnight exercise changing the country.”

The news conference on Saturday came after he chaired the first meeting of the new Cabinet.

He said he had told his ministers “exactly what I expect of them in terms of standards, delivery, and the trust that the country has put in them”.

The Prime Minister said it had been a “moment in history” on Saturday morning as some of his top team received their privy seals, which was followed by a meeting in Number 10.

Sir Keir said: “At that meeting, I had the opportunity to set out to my Cabinet precisely what I expect of them in terms of standards, delivery and the trust that the country has put in them.”

He said they had also discussed “mission delivery, how we would put into action the plans that we had set out in our manifesto”.

Sir Keir will make his debut on the international stage as Britain’s premier when he flies to Washington DC for the Nato gathering next week, which is expected to include discussions on support for Ukraine.

He told the news conference: “It is for me to be absolutely clear that the first duty of my Government is security and defence, to make clear our unshakable support of Nato.

“And of course to reiterate, as I did to President Zelensky yesterday, the support that we will have in this country and with our allies towards Ukraine.”