Sir Keir Starmer has insisted the G20 still matters and is a “really important” forum to bang the drum for British business, despite the decision of Donald Trump to boycott the international summit in South Africa.

Asked what he thought of the US president’s decision, the prime minister simply said Mr Trump had “set out his position”.

The PM added he thought it was “really important to be [at the G20] to talk to other partners and allies so we can get on with the discussions around global issues that have to be addressed, and do have an impact back at home, but also to take the opportunity face to face to further the deals that I want to do for our country”.

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Sir Keir has faced heavy criticism at home for the amount of time he has spent overseas and focusing on international affairs. His trip to South Africa to attend the G20 summit is the 45th country the prime minister has visited since taking office.

Speaking to journalists on the flight over to Johannesburg, Sir Keir defended his decision to fly out days before a difficult budget, saying that the international issues being discussed in South Africa have an impact at home, while the G20 nations are important to Britain’s economy.

“The G20 are the 20 strongest economies in the world, they are very important to the UK,” he said.

“In the last three years, the jobs that have been generated in the UK from countries in the G20 is 200,000 and that focus in the budget will be very much the economy and the cost of living. I will focus on the deals we can do, the business we can do with our partner countries and make sure that the work we do internationally is impacting directly at home in the positive sense, that if you want to deal with the cost of living and make people better off, good, secure jobs with investment from G20 partners and allies is really important.”

This summit is the first one in the G20’s 26-year history that a US president has not attended, with one diplomatic source acknowledging this was raising serious concerns. They said: “Trump also made the argument that the G7 should be the G8 [at the last meeting in Canada in June] and now he’s not even going to the G20, so his lack of attendance is, of course an issue.”

Mr Trump has also ordered US officials not to travel to South Africa for the annual meeting, although the country’s president, Cyril Ramaphosa, said on Thursday evening that this might change, with discussions now under way with the US.

While Mr Trump is not attending, Sir Keir will leave the G20 summit early, coming back to the UK on Sunday to prepare for a tax-raising, and possibly manifesto-breaking budget on Wednesday.

The chancellor raised £40bn in taxes in the last budget, insisting that this was a “once in a parliament” tax raid. A year on, Rachel Reeves now has to raise billions more as she looks to fill a black hole as much as £30bn in the public finances, driven in part by a downgrade in productivity, which has lowered growth forecasts, and also her reversal on spending cuts – the winter fuel allowance and disability benefits – that has left her with around £7bn to find.

The government has U-turned on its plan to raise income tax but is expected to extend a freeze on tax thresholds by two years from 2028. The measure will raise about £10bn in additional tax as workers find themselves dragged into higher tax bands and has led to accusations that Labour has broken its manifesto pledge not to raise taxes on working people.

The prime minister, asked whether everyone should expect tax rises in the budget on Wednesday, refused to answer directly. Instead, he said it would be “a Labour budget with Labour values” and based on “fairness”.

He added: “It will have absolutely in mind protecting our public services, particularly the NHS, cutting our debt, and dealing with the cost of living, bearing down on the cost of living. So, they’ll be the principles that will run through the budget.

“Now, of course, the right decisions have to be taken. And we have to see this in the context of 16, 17 years now where we’ve had the crash in ’08, followed by austerity, followed by a not very good Brexit deal, followed by Covid, followed by Ukraine, and that’s why we have to take the decision to get this back on track.

“I’m optimistic about the future, I do think if we get this right, our country has a great future. They’ll be the principles behind the budget.”

While the prime minister is focusing on trade at the G20 summit, Ukraine will also be on the agenda amid reports the Trump administration and Russian officials have drawn up a new peace plan to end the war there.

It would require major concessions from Kyiv, including giving up territory not currently occupied by Russia to Moscow and halving the size of the Ukrainian army. The deal has reportedly been drawn up by Mr Trump’s special envoy Steve Witkoff, who met the current secretary of the national security and defence council of Ukraine and former defence minister, Rustem Umerov, in Miami.

I’m told by one diplomatic source that the Europeans have yet to see this plan, while there are questions as to how advanced in the US administration these proposals are and whether they have the support of Secretary of State Marco Rubio.

European diplomats are stressing that any peace talks have to involve both Ukraine and European input if it is to have any hope of working. Kaja Kallas, the EU’s foreign policy chief, said on the eve of the G20 summit they are yet to see any concessions on the Russian side.

“We welcome all meaningful efforts to end this war, but like we have said before, it has to be just and lasting,” she said. “That also means that the Ukrainians, but also the Europeans, agree to this.”

European leaders are discussing how to best equip Kyiv for another winter of war. Talks are expected to continue this weekend over the plan to use Russia’s frozen assets to generate a €140bn loan for Ukraine.

The plan is currently stalled over Belgium’s concerns of legal risk in releasing funds from the Brussels-based depository Euroclear, where most of the Russian assets are held.