Sir Keir Starmer has warned his new MPs many voters are lending Labour their support and are “not converts”.
The new Prime Minister posed for an historic picture with his 411 MPs on Monday.
And he is understood to have warned them against complacency.
Sir Keir 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.”
The MPs gathered on Monday evening at Church House in central London for the first meeting of the new Parliamentary Labour Party.
Economists have warned that the new Labour government will likely have little choice but to introduce tax rises to rebuild struggling public services.
In the run up to the election, Paul Johnson, director of the Institute for Fiscal Studies (IFS), criticised all parties for a “conspiracy of silence” in the manifestos over the true scale of the economic and fiscal challenges facing the new administration.
Analysts at the investment bank Citi have said they expect a package of tax increases in the autumn worth around £15 billion.
The IFS has warned that wealth taxes, such as inheritance tax and capital gains tax, are one of the few options left on the table for a Labour government.
On the campaign trail, Labour ruled out increases to income tax, National Insurance, the headline rate of corporation tax and VAT, which combined make up around three quarters of the Treasury’s tax take.
She has also come under pressure to tax interest paid to banks on reserves that they are required to deposit with the Bank of England. Mr Brown has suggested that introducing a tax could raise £1.3 billion.
However, Ms Reeves said: “I’ve got no intention on changing the way the reserves are treated and the interest paid on those reserves.”
