Liz Truss repeatedly declined on Sunday to spell out her plans to tackle soaring energy bills, 48 hours before she is tipped to become the next prime minister.

The Foreign Secretary, who will find out on Monday if she has defeated rival Rishi Sunak in the Tory leadership contest, promised to act “immediately” to tackle the energy crisis upon entering Number 10 but would not offer clear detail of the support struggling households can expect.

There have been ever-louder calls in recent weeks for the Government to intervene to support the most vulnerable, with energy bills set to rise to around £3,500 this winter for the average household.

Ms Truss, who denied she was being “coy”, said she wanted to reassure voters that help is coming but indicated they would need to wait for a few days yet to find out what exactly such support might look like.

In an interview with the BBC’s Sunday with Laura Kuenssberg programme, she said: “I will act if I’m elected as prime minister. I will act immediately on bills and on energy supply because I think those two things go hand in hand.

“We need to deal with the immediate problem, we need to help people. We need to help businesses. But we also need to sort out the supply issues.”

Repeatedly pressed on what form that action would take, Ms Truss said: “Before you have been elected as prime minister, you don’t have all the wherewithal to get the things done.

“This is why it will take a week to sort out the precise plans and make sure we are able to announce them. That is why I cannot go into details at this stage. It would be wrong.

“I understand people are worried and I want to reassure people that I am absolutely determined to sort out this issue as well within a month, present a full plan for how we are going to reduce taxes, how we’re going to get the British economy going, and how we are going to find our way out of these very difficult times.”

Mr Sunak, widely believed to be heading towards a defeat, took a similar approach as he told the programme he could not give the “exact” pennies or pounds of support.

“I haven’t seen all the numbers and nation’s finances.”

Labour leapt on the lack of detail, with shadow attorney general Emily Thornberry labelling it “extraordinary” that Conservative Party leadership candidates “cannot give a specific answer to the one question frankly that everybody wants an answer to”.

Some experts and analysts have warned that neither Ms Truss nor Mr Sunak have set out sufficient plans to shepherd the UK through the coming months as the country braces for soaring energy bills and worsening inflation.

Ms Truss’ proposals, which include reversing April’s rise in national insurance and next year’s corporation tax increase from 19% to 25%, have prompted accusations they could worsen the already grim economic situation.

On Sunday, Scottish First Minister Nicola Sturgeon offered a stark warning to the likely next prime minister, telling Sky News: “If she governs how she has campaigned over the summer, she will be a disaster.”

Ms Truss appeared to brush off such a warning as she insisted that the UK had “been through worse”.

“With me, what you see is what you get. I don’t make promises I can’t keep. I follow through on what I’ll say I’ll do.

“I will be clear with the public about what we are going to face and there will be challenging circumstances, there’ll be difficult decisions to be made.

“Not all of those decisions will be popular but I will be honest about what we will have to do.

“I’m also somebody who is positive. I’m clear that we can deal with these issues, that Britain has been through worse, frankly, in the past. We have the capability, we have the attitude and we have the spirit to deal with the challenges.”

Recent days have been full of speculation about who could make up the Cabinet in a Truss administration, with Business Secretary Kwasi Kwarteng tipped to become her chancellor.

The Sunday Times also reported that the energy crisis will feature centrally in Ms Truss’ first speech from Downing Street, if she becomes prime minister.

There is also a growing expectation that the next prime minister will make an early visit to Kyiv to shore up support for Ukraine.

It comes as Mr Johnson prepares to leave office with praise from Volodymyr Zelensky ringing in his ears.

The Ukrainian leader, writing in the Mail on Sunday, called the outgoing prime minister a “true friend” as he paid tribute to his steadfast support for the war-torn country.

Mr Johnson used his own article in the Sunday Express to reflect on his achievements while also urging his party to come together after a summer of in-fighting and “back that new leader wholeheartedly”.

In what is set to be a frenzied few days for UK politics, Mr Johnson’s successor will be announced on Monday, taking over as prime minister the following day.

Mr Johnson and his successor will go to Balmoral, rather than Buckingham Palace, for the appointment of the new prime minister on Tuesday, in a break from tradition.

The Queen will receive Mr Johnson on Tuesday at her Aberdeenshire home, where he will formally tender his resignation.

This will be followed by an audience with the new Tory leader, where she or he will be invited to form a government.

It is likely to be a “very sad” occasion for Mr Johnson, according to his former chief of staff and close aide Lord Udny-Lister.

But he also told Sky News that he would “never say never” about a return for Mr Johnson.

“He is going to be watching all this and if something happens in the future, as you said, the ball comes loose in the scrum, then anything can happen. I’m not going to predict. All I’m saying is I would never write him off.”

Other senior figures in the Conservative Party were quick on Sunday to offer advice and warnings to the next prime minister.

Former chancellor Lord Hammond admitted concerns for his party’s reputation for “sound government,” as he appeared on Sky News to warn that the incoming Government must be based on the principles of “sound money, balanced budgets, paying our way in the world”.

The new leader will also inherit what veteran Tory MP David Davis labelled as “probably the second most difficult brief” facing any post-war prime minister.

“The only person with a worse brief coming in was Margaret Thatcher,” the former minister told Sky News as he urged the next leader to put ideology to one side to meet the scale of the crisis.

Sir John Redwood echoed concerns about the economic challenge but the Thatcherite MP, who some have tipped as in line for a Government job under Ms Truss, said that Mr Sunak’s prescription was wrong.

“I think there is a better way when you should be trying to shield people from a long and deep recession, and understand that inflation will come down as that economic slowdown develops,” he told Times Radio.