Andy Burnham said he had still not decided who will be in his cabinet ahead of him taking over from Sir Keir Starmer as prime minister on Monday.
Asked why he had not announced his top team, the new Labour leader said: "It would be somewhat premature and would, I think, cause complete chaos if you start half a reshuffle before you're in the position."
There has been widespread speculation about Burnham's cabinet picks, with Ed Miliband and Shabana Mahmood thought to be in the running to replace Rachel Reeves as chancellor.
In an earlier speech after becoming party leader, the former mayor of Greater Manchester outlined his vision for government including reforms to the social care system.
Reform UK leader Nigel Farage said Burnham "comes in with absolutely no mandate of any kind at all" and called for "an immediate general election so the country can decide the future".
But Lib Dem leader Sir Ed Davey said he welcomed Burnham's call for a more collaborative politics, adding "my door is open".
He said the Lib Dems would particularly want to see changes to the water industry, support for the NHS and changes to social care, including more help for family carers.
Burnham, who returned to Parliament a month ago in a by-election, cemented his status as the sole leadership candidate after being backed by 379 Labour MPs, as well as all 11 trade unions affiliated to the party, earlier this week.
He will take over from Sir Keir as prime minister on Monday, following a reception with King Charles III.
Asked by reporters about who will be in his cabinet, Burnham said: "I am finalising those decisions, and I will come to conclusions very shortly, and then I will announce those on Monday."
In his first speech as Labour leader at the TUC headquarters in central London, he said his appointments for cabinet would reflect "all parts of our party" and "all communities".
He said his government would give "hope back" to working-class communities who the political class had "turned its back on", reiterating his plan to devolve power away from Westminster to the regions.
Devolution has been one of Burnham's central messages since he announced his bid to replace Sir Keir.
He said his government would deliver the biggest change in decades, rejected the "neoliberal" policies of the past 40 years, including when he was in government, and vowed to reindustrialise the country.
And in a message to Labour MPs, he said he would work relentlessly to build a culture of "one Labour team".
"We won't beat Britain's new right if we are consumed by infighting and pulling in different directions," he said.
He added that his approach to governing would be characterised by "problem-solving rather than point-scoring".
He added that he knows what he believes and what he wants to do in government, telling the audience: "I have a plan."
Social care is 'broken'
His speech was light on policy details, which are expected to come as he moves into Downing Street next week.
But he told reporters a top priority would be reforming the social care system, pledging to "expend quite a lot of political capital" on what he said had been a long-neglected sector.
Social care is devolved, meaning any reforms Burnham will look to implement will only apply to England.
Governments and opposition parties have long tried to grapple with the issue of how social care should work and be funded.
Unlike the NHS , social care is not free at the point of use and high costs sometimes mean people are forced to sell their homes to pay for what they need.
Burnham has long called for reform. As health secretary under Gordon Brown, just weeks before the 2010 general election, he published a white paper calling for the creation of a National Care Service which would be free at the point of use.
"My dad has Alzheimer's, and you see what the staff have to put up with. It's not their fault. The system's broken." he told reporters.
"You can't leave social care like this. And this is part of my criticism of Westminster. It has flinched over the years from these big decisions."
'Get this right'
Conservative Party chairman Kevin Hollinrake said Burnham should recall Parliament, which is now in recess until September, to explain his plan to MPs.
"It's not too late for Andy Burnham to do the right thing, make a statement, and answer questions from members of Parliament on Monday about his 'plan'," he said.
Linda Hobson, from Unison, one of the unions linked to Labour, said Burnham "must get this right and bring back the hope he promises".
Having run twice before to become Labour leader, Burnham's successful bid this time round marks a rapid rise to power following the former Greater Manchester mayor's by-election win in Makerfield last month.
Labour has trailed Reform UK in opinion polls for almost 18 months and supporters will be hoping Burnham's arrival as prime minister will reverse their fortunes.
Sir Keir resigned as Labour leader last month, saying he accepted he was not best placed to lead his party into the next general election.
Heavy Labour losses in May's local elections followed by Burnham's by-election victory in Makerfield had left Sir Keir facing widespread calls from his own MPs for him to stand aside and allow Burnham to replace him.
Burnham paid tribute to Sir Keir on Friday, celebrating his record in government including improving rights for workers and renters, seeing NHS waiting lists fall and bringing rail back under public control.
Source: BBC
--Agencies
