Former prime minister Gordon Brown has said Labour can only deliver on its principles in power.
"Leaders come and leaders go, I know that," Mr Brown said at London's Southbank Centre as he began his speech with a call for the party to continue its "mission" to "advance social justice".
"We can only measure the progress we make by the difference we make in people's lives," he added.
"It is not an abandonment of principles to seek power and to use that power in government. It is the realisation of principles."
The newly elected Scottish Labour leader has said the long and increasingly petty campaign to replace Ed Miliband is letting David Cameron off the hook.
Kezia Dugdale, who was elected yesterday, said she was concerned that the party leadership contest south of the border would hinder its ability to be an effective opposition in the meantime.
The thing that bothers me about that is not so much the petty exchanges, but the fact that David Cameron is getting off the hook all through the summer.
I think he's having three summer holidays, and he can do so in the blissful knowledge that the Labour Party will continue to talk about its own future rather than scrutinise his government's record and what they plan to do in the future. That's quite worrying.
Dugdale also questioned Jeremy Corbyn's willingness to be prime minister, but acknowledged that his popularity and "big ideas" are exciting the country.
Earlier this month, she had questioned how "a guy that's broken the whip 500 times" can enforce party discipline, in an interview with The Guardian.
Jeremy Corbyn will today set out his plans for "better business" promising to stand up for small businesses and clamp down on corporate tax avoiders.
It comes as former prime minister and chancellor Gordon Brown prepares to give a speech on the future of the Labour Party this afternoon.
Among the proposed policies are a freeze on tax rates for small businesses, a national investment bank, fair rents for local shops and more resources for HMRC to tackle corporate tax avoidance.
The current government seems to think 'pro-business' means giving a green light to corporate tax avoiders and private monopolies.
I will stand up for small businesses, independent entrepreneurs, and the growing number of enterprises that want to cooperate and innovate for the public good.
A poll rates Jeremy Corbyn as the leadership candidate both most likely to improve Labour's fortunes and damage the party's chances.Read the full story ›
Gordon Brown will make a high-profile intervention in the Labour leadership contest with a keynote speech at London's Southbank Centre.
While he is not expected to endorse any of the four candidates in the race, Mr Brown's address is expected to distance himself from the policies of front-runner Jeremy Corbyn.
The speech on "power for a purpose" is expected to call for Labour to realign itself on the economy.
Mr Brown's intervention comes after several public attempts by his predecessor in power Tony Blair to warn against a Corbyn victory.
Gordon Brown has warned that Nicola Sturgeon's plans would leave a £7.6 billion hole in Scotland's economy, as he pitched Labour as the country's "party of fairness and social justice".
In a letter to voters, the former Prime Minister claimed both the SNP and the Conservatives would leave the NHS at risk.
He said the nationalists would "scrap the Barnett formula, meaning that our NHS and other public services could only be funded by taxes raised in Scotland."
He also warned that the Conservatives were "committed to billions of pounds more austerity - which means less money for Scotland and our NHS".
In a speech later Gordon Brown is expected to say it is time to 'reset politics' beyond the 'old issues of constitutional change'.-
It is time to move beyond two years of constant talk of constitutional change to a new focus in the next two years on the social and economic change that Scottish people have said they want.
I am pressing the reset button because it is time to move beyond the old issue of bigger powers for the Scottish Parliament, as we now have more powers than at any time, to the issue that really concerns Scotland - better lives for the Scottish people.
The Kirkcaldy and Cowdenbeath MP is to address the annual meeting of Scottish Labour councillors in Glasgow this morning.
Gordon Brown hopes to "reset" Scottish politics in a bid to shift the focus from the constitutional debate to how to improve the lives of ordinary people.
Independence has dominated politics north of the border for the last two years ahead of September's historic referendum.
Former prime minister Mr Brown played a pivotal role in the campaign to keep Scotland in the UK and was also a key player in the pledge by the Westminster parties to deliver more powers for Holyrood.
Former prime minister Gordon Brown and key 'Better Together' campaigner is to announce that he is standing down as an MP, according to reports.
Mr Brown, 63, will soon confirm that he will quit at the general election in May, after playing a key role in the Scottish independence referendum campaign, sources have said.
The Labour MP has focused on charity work and his role as United Nations special envoy for global education since his resignation as prime minister in 2010.
An ally told the Sunday Mirror newspaper: "Gordon has confirmed to friends that he will stand down at the election in May.
"He wants to go out on a high after effectively salvaging the campaign to keep the UK together in September. He will focus on his charity work."
Mr Brown was first elected to Parliament in 1983 and was prime minister from 2007 to 2010 and chancellor from 1997 to 2007.
His last-minute intervention in the referendum debate was widely credited with helping the pro-union Better Together campaign to victory.
Gordon Brown will not enter the Scottish Labour leadership race, a source close to the former prime minister has confirmed.
Brown was tipped as a possible replacement for Johann Lamont after her resignation last week.
Some party members suggested Brown was a unifying figure, highlighting the work he did for the No campaign during the run-up to the Scottish referendum.
"For the past four years, and on every occasion he has been asked, Mr Brown has made it clear he is not returning to frontline politics, a source close to Brown said. That position has not changed."