Keir Starmer has told his new MPs they must work relentlessly to convince people who voted Labour to back them again at the next election.
It came as the new Prime Minister welcomed a massive haul of new MPs to Westminster after Labour's landslide victory last week. A whopping 412 Labour MPs have been elected - the second largest number in history.
Earlier new Chancellor Rachel Reeves has warned the UK economy is in its worst state since the Second World War and blamed previous governments for "decisions deferred and decisions ducked." But her keynote speech outlined how she aims to get the economy firing again, while trying to repair battered public services without pouring in loads of cash.
Read the Chancellor's FOUR key takeaways, click HERE
It is understood Mr Starmer told his colleagues gathered in Westminster that voters "placed their trust in us for now, but they are not converts". He said: "Election victories do not fall from the sky. They are hard won and hard fought for and this one could only have been won by a changed Labour party. We campaigned as a changed Labour party – and we must govern as a changed Labour Party."
Repeating his message from the steps of No10 last week, he added: "We have to return politics to public service, to show to the British people that politics can be a force for good. We have a mandate for change, a mandate for renewal."
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The MPs gathered on Monday evening at Church House in central London for the first meeting of the new Parliamentary Labour Party. One source told The Mirror there wasn't a room "big enough" on the Parliamentary estate for the 412 MPs to gather together for a joint photograph.
New members of the Labour Government, including Education Secretary Bridget Phillipson, Transport Secretary Louise Haigh, and Business Secretary Jonathan Reynolds, were seen leaving their ministerial cars to attend the meeting.
Re-elected MP for Bradford West Naz Shah told The Mirror: "The grown-ups are back in charge. It's not a playground, this is running the country and hitting the ground running." Leaving the meeting, she said the central message of Mr Starmer's address to MP was about getting back to "serving people". She added: "The change was so profound."
The group of new MPs includes 22-year-old Sam Carling, the new “Baby of the House”, who was elected in North West Cambridgeshire. Dr Zubir Ahmed, a consultant surgeon who won in Glasgow South West, is one of 37 Labour MPs who won seats in Scotland.
Colonel Al Carns, who won the Military Cross for services in Afghanistan, is among the over a dozen veterans to be elected as Labour MPs last week. Dr Lauren Sullivan, the new MP for Gravesham, is a scientist who has been working to eradicate malaria. Josh MacAlister, who founded a groundbreaking scheme to get graduates into social work, won in Whitehaven and Workington.
The economist Miatta Fahnbulleh also joined the group as the new MP for Camberwell and Peckham - the seat vacated by the veteran Labour MP Harriet Harman.
Hamish Falconer was a diplomat at the Foreign Office for around a decade before fighting and winning the seat of Lincoln from the Conservatives last week. He is the son of Charlie Falconer - a former flatmate of Tony Blair, who served as a Cabinet Minister in the former Labour Prime Minister's Government.
MPs returning to Parliament also include former Cabinet minister Douglas Alexander, who has been appointed as a Business Minister. Heidi Alexander, who left the Commons in 2018, also returned last week as the new MP for Swindon South and was swiftly appointed a Minister in the Ministry of Justice.
Many new MPs have been sharing photographs on social media of them making their journey to Westminster. Tris Osborne, a former teacher and now the Labour MP for Chatham and Aylesford, posted an image of his school, saying: "Saying goodbye to my classroom this morning".
Mr Starmer completed a whistlestop trip around the four nations of the UK today - with a final visit to Wales, promising a "reset" between Westminster and devolved nations. He will fly to Washington DC on his first foreign trip on Tuesday. He’s expected to meet world leaders including US President Joe Biden at a Nato summit.