Sir Keir Starmer insisted he does not want to get involved with people’s personal life choices when questioned on whether he wants to persuade people to have more children.
The issue was put to the prime minister amid growing concern the birth rate in the UK, and many Western countries, is leading to population decline and long-term issues with an ageing population.
But when asked if he had any ideas on how to persuade people to have more children in Britain and if he thought the birth rate needed to rise, it provoked an impromptu laugh from Sir Keir during a flight to Washington.
The prime minister had been asked: “What can you do?”
He responded: “I think there are a number of ways of growing the economy. When I said local growth plans I actually had in mind plans led by local mayors and businesses, not a birth plan.
“I’ve spent my whole time saying I’m not going to tell people how to live their lives. I’m not going to start by dictating whether they should or shouldn’t have children.”
But the episode has provoked fury among Tories on the right, who have been arguing that birth rate decline is a long-term structural problem.
They have taken inspiration from right-wing Hungarian prime minister Viktor Orban’s policies to encourage people to have large families.
They also questioned whether a prime minister who wants to ban smoking in beer gardens really wants to “not tell people how to live their lives”.
Former Tory MP Sir Jacob Rees-Mogg, who has six children, said: “Starmer seems incapable of telling the truth. He spends his whole time telling people how to live their lives with smoking bans, advertising bans and potentially sugar and salt taxes.
“No one is asking him to tell people how many children to have, all that is being suggested is that he should say having children is a good thing but the old lawyer never wants to give a straight answer.”
Former Tory MP Miriam Cates said: “It’s very concerning that the prime minister doesn’t appear to understand the basic mathematical problem underpinning the UK’s sluggish growth; namely that falling birth rates mean we have fewer and fewer working people supporting growing numbers of pensioners.
“No amount of local business plans or productivity drives are going to change this fundamental fact.
“If the prime minister wants growth, he should follow the example of increasing numbers of Western leaders urgently looking for ways to increase the birth rate.”
Currently, the UK has one of the lowest birth rates in the Western world at 1.56 children per woman, compared to 1.66 in the USA and 1.83 in France.
The rate of 10.4 births per 1,000 population is the lowest it has been in the UK since records began, with the number peaking at 21.2 in 1947 just after the Second World War.
The picture of the prime minister laughing at the birth rate question had been wrongly used by political opponents, including the Conservatives, on their Twitter account claiming he was laughing at cutting winter fuel payments.