To try and press home his advantage, Sir Keir was on Monday carrying out a shake up of his top team with former Work and Pensions Secretary Yvette Cooper returning to the Shadow Cabinet in a senior role.
Cooper has been appointed shadow home secretary.
A poll by Opinium earlier this month showed the ruling Conservative Party had fallen behind Labour for the first time in a year, as the Prime Minister battled a rebellion from his own backbenchers over his handling of the Owen Paterson sleaze affair.
Watch: Rayner appears blindsided in Labour reshuffle
Since then Mr Johnson has been hit by further unrest over social care reforms and the HS2 rail project.
But Sir Keir’s second reshuffle inside a year risked being overshadowed by tensions with Labour’s Deputy Leader Angela Rayner who appeared to be caught off-guard by the timing of the move.
Delivering a speech at the Institute for Government think-tank on Monday morning, on Labour’s five point plan to shake up Parliamentary standards following the Paterson affair, Ms Rayner told journalists she was not aware of any reshuffle.
But Party officials have played down suggestions she was out of the loop, insisting that Ms Rayner had been briefed by Sir Keir prior to delivering her speech on standards.
Asked about reports of the reshuffle, Ms Rayner said: "I've been concentrating on the job that I'm doing."
In the first major change, Jo Stevens confirmed she had been moved from her current role as Shadow Culture Secretary to replace Llanelli MP Nia Griffith as shadow Welsh Secretary.
“It’s been a privilege to closely work with hugely talented people in the DCMS sector, the fastest growing before the pandemic & one of the hardest hit,” she said on Twitter.
“I want to continue the great work of my friend @NiaGriffithMP in the new job.”
Meanwhile Cat Smith, MP for Lancaster and Fleetwood, announced she would be standing down from her role as Shadow Secretary of State for Young People and Democracy and returning to the backbenches.
However in a letter published on Twitter she warned Sir Keir about the "damage" Labour was suffering because former leader Jeremy Corbyn is not being re-admitted to the party.
Her letter said: "You will be aware that we had a meeting scheduled for later this week, during which I wanted to raise the issue of, and my concerns about, Jeremy not being readmitted to the Parliamentary Labour Party after he was readmitted into our party membership following due process.
"This position is utterly unsustainable and it is important that you truly understand how much damage this is causing in Constituency Labour Parties and amongst ordinary members, a number of whom are no longer campaigning."
Shadow attorney general Lord Falconer is stepping down from Labour’s frontbench team, it has been announced.
In a letter to party leader Sir Keir Starmer, the peer said: “As we have discussed, I think the time has come for me to step back from the role of shadow attorney general.
“I was 70 last Friday and I will continue to work on the number of political areas that I have a personal interest in, including assisted dying and the wider constitutional issues facing the UK.
“I have throughout been a close adviser to you and will, as you have asked, continue to be so. I strongly supported your leadership of the Labour Party and will continue to do so.”
In reply, Sir Keir said he was sorry Lord Falconer was stepping down.
“I am glad that you will continue to be engaged in frontline politics and that you will remain a close adviser to myself and this shadow cabinet,” he said.
In other moves, Lisa Nandy will face off against Michael Gove as shadow levelling up secretary and will be replaced as shadow foreign secretary by David Lammy.
There are big promotions for Bridget Phillipson and Wes Streeting who take on the roles of shadow education secretary and shadow health secretary.
Jonathan Ashworth, who has had the health brief through the pandemic, is moved to be shadow work and pensions secretary.
Former leader Ed Miliband becomes shadow climate change secretary while Jonathan Reynolds takes on his former portfolio of business, energy and industrial strategy.
Shadow Cabinet unveiled:
Deputy leader, shadow first secretary of state, shadow chancellor of the Duchy of Lancaster and shadow secretary for the future of work: Angela Rayner
Shadow chancellor of the Exchequer: Rachel Reeves
Shadow business and industrial strategy secretary: Jonathan Reynolds
Shadow climate change and net zero secretary: Ed Miliband
Shadow levelling up, housing, communities and local government secretary: Lisa Nandy
Shadow defence secretary: John Healey
Shadow digital, culture, media and sport secretary: Lucy Powell
Shadow education secretary: Bridget Phillipson
Shadow environment, food and rural affairs secretary: Jim McMahon
Shadow foreign, commonwealth and development affairs secretary: David Lammy
Shadow health and social care secretary: Wes Streeting
Shadow home secretary: Yvette Cooper
Shadow international trade secretary: Nick Thomas-Symonds
Shadow minister of state at the Cabinet Office: Jenny Chapman
Shadow justice secretary: Steve Reed
Shadow transport secretary: Louise Haigh
Shadow women and equalities secretary, party chair and chair of Labour Policy Review: Anneliese Dodds
Shadow work and pensions secretary: Jonathan Ashworth
Shadow Attorney General: Emily Thornberry
Shadow cabinet minister for mental health: Rosena Allin-Khan
Shadow cabinet minister for international development: Preet Gill
Shadow chief secretary to HM Treasury: Pat McFadden
Shadow Wales secretary: Jo Stevens
Shadow Scotland secretary: Ian Murray
Shadow Northern Ireland secretary: Peter Kyle
National campaign coordinator: Shabana Mahmood
Shadow leader of the House of Commons: Thangam Debbonaire
Shadow chief whip: Alan Campbell
Shadow leader of the House of Lords: Angela Smith
Opposition chief whip in the House: Roy Kennedy
Watch: Labour calls for stricter Covid measures