Britain's new Prime Minister Rishi Sunak earned cheers from his lawmakers and plaudits from political commentators on Wednesday as he locked horns with the opposition Labour Party in parliament for the first time since becoming leader.

But he delayed the announcement of a keenly awaited plan for repairing the country's public finances until November 17, two-and-a-half weeks later than previously planned.

The postponement, Sunak's first policy decision since taking over from Liz Truss on Tuesday, briefly raised British borrowing costs in financial markets but there was no repeat of the panic bond-selling caused by Truss's September tax-cutting plan.

Sunak, seeking to end a period of political instability that has seen two previous prime ministers leave Downing Street in two months, struck a confident tone as he sought to get his Conservative Party off the back foot.

The 42-year-old – who became the youngest prime minister in modern British history, and the first non-white ever to hold the post, when he was appointed by King Charles on Tuesday – hit back at Labour's claims that he is planning to cut public spending sharply to repair the public finances.

"My record is, when times are difficult in this country I will always protect the most vulnerable," Sunak said to vociferous support from the Conservative benches in the House of Commons. "We did it in COVID-19 and we will do that again."

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He sought to fend off Labour attacks about his unexpected decision to reappoint a key figure from the party's right wing, Suella Braverman, as interior minister – a week after she was fired for a security breach – and he kept his cool when Labour raised the issue of rich foreigners paying little tax.

Sunak's wife – the daughter of an Indian billionaire – earlier this year said she would start to pay tax in Britain on her foreign income after media reported she had "non-dom" status at a time when Sunak was raising taxes for households.

The mood on the Conservative benches was transformed from last week, when Sunak's predecessor, Liz Truss, battled through her third and final Prime Minister's Questions (PMQs) after her credibility was sunk by a series of U-turns.

"Vibe shift between last week's PMQs and today is (once again) remarkable," Sebastian Payne, the Whitehall editor of the Financial Times, wrote on Twitter. "Tory benches are full of noise and glee, unlike totally leaden mood last week."

Sunak's Conservatives saw their standing in opinion polls sink to multi-year lows as Truss stumbled through her six weeks as prime minister. Sunak has ruled out calling an early national election; one must take place by January 2025.