The outgoing leader will hold farewell talks with his senior ministers in Downing Street before taking his last Commons grilling at the despatch box at noon on Wednesday.
Meanwhile, prime minister-in-waiting Burnham will edge closer to No 10 as nominations for the Labour leadership formally close.
The former Greater Manchester mayor's premiership has already been cemented after he received the backing of 369 of the party's 403 MPs, making it mathematically impossible for a rival to enter the contest.
Under Labour rules, candidates need the backing of 81 MPs to stand in a contest, meaning he is set for a coronation.
Starmer, who will leave No 10 on Monday, told a reception attended by campaigners whose causes he had supported that they had "made me who I am".
He is spending his final days in office highlighting what he sees as his legacy - support for Ukraine and championing domestic campaigns like the Hillsborough Law.
"I'm pleased to have delivered on the promises that I made to many people in this garden, and I'll make this last promise, which is I will stand with you and walk with you for as long as I've got breath in my body," he said.
In a sign of the closer relationship he has tried to forge with European neighbours, he also attended Bastille Day celebrations in Paris on Tuesday with Emmanuel Macron, who awarded him the Legion d'honneur - France's highest national honour - in recognition of his work with France on European security.
"Prime minister, dear Keir, I wanted to reiterate my gratitude and the gratitude of the French people, obviously for your years as a prime minister," Macron said, according to a statement released by Starmer's office.
"But I have to say, beyond that, for your personal leadership and your commitments for obviously your country, but the security of our Europe, Ukraine, the bilateral relationship, your decency."
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