Tom Watson has been elected deputy leader of the Labour Party following a three-month campaign.
He beat four other contenders for the job - Ben Bradshaw, Stella Creasy, Angela Eagle and Caroline Flint.
Mr Watson will serve under veteran left-wing MP Jeremy Corbyn, who was named party leader with 59.4% of the vote in the first round.
He promised to back the leader 100%, adding "only through unity comes the strength we need to fight the Tories".
In his victory speech, Mr Corbyn thanked his new deputy, adding: "Tom is passionate about communication, passionate about holding the state, and unaccountable people who don't wish to be accountable, to account.
"Tom is your man to do that."
West Bromwich East MP Mr Watson, 48, has been an MP since 2001.
He is a former junior minister and ally of former prime minister Gordon Brown, and is known for campaigning on issues including phone hacking, the government's surveillance laws and historical child abuse.
Mr Watson quit as Ed Miliband's election campaign co-ordinator in 2013 after a candidate selection row.
'Nothing prepares you'
Mr Watson emerged as the winner in the third round of voting. He received 160,852 votes - 39.4% - in the first round, 170,589 - 42.2% - in the second round, and 198,962 - more than 50% of the vote - in the third round. Ms Creasy came second with 26% and Ms Flint third with 22%.
There was a standing ovation and huge cheers as his victory was announced.
Invited to the stage to speak, he said: "Nothing really prepares you for this moment - and for those of you that don't know me that well, being prepared is not something I'm renowned for."
He said he had written his victory speech backstage with his children, who told him to thank his mum and dad. Mr Watson also thanked Baroness Alicia Kennedy, his campaign director, adding it was "impossible to reflect on all the people who had made it happen".
"I gave you my solidarity as a five-year-old and I thank you for returning it back in this selection," he said, to applause and laughter.
He went on: "Be in no doubt, in the Tories' second term, Labour is the last line of defence for the millions of people who suffer in their hands.
"Only Labour can speak for the real Britain."
Mr Watson said there "was only one Labour" and it was "bigger" than its leaders, members and supporters.
He said: "On behalf of the millions who need us, we are the guardians of decency, fairness, justice and equality in the United Kingdom...
"We articulate and embody the common sense compassion of the British people.
"The no-nonsense belief that things ought to be fair - if you put in what you can you should get out what you need."
Career in brief
- West Bromwich East MP 2001-present
- 2005-06: Government whip
- 2006: Defence minister
- 2007-08: Assistant government whip
- 2008-09: Cabinet Office minister
- 2011-13: Labour deputy chairman
- 2011-2013: Labour elections co-ordinator
- 2015: Labour deputy leader
Mr Watson said he believed Labour remained the "true party of Britain" but it needed to "think again about what we offer and how we communicate it".
The conference applauded as Mr Watson warned the Conservatives to "watch your backs", insisting Labour could win in 2020.
Len McCluskey, general secretary of the UK's biggest union Unite, congratulated both men, saying: "The task now for all of us who support Labour is to back the leadership team, to unite, to turn to face the Tories and hold them to account.
"It is what the voters expect, it is the way back to power and it is the duty of those at all levels of the party to deliver."
More than 400,000 people voted in the deputy leadership election - a turnout of 73.8%.
The results were announced at a special conference in central London by Jim Kennedy, chairman of Labour's national executive committee.