Boris Johnson has insisted the public “will understand” cuts to overseas aid as it emerged the UK’s contribution to a United Nations sexual and reproductive health agency had been cut by 85%.

The Prime Minister said on Thursday that aid spending would return to 0.7% of national income when it was “fiscally prudent to do so” after he broke a manifesto commitment to slash spending to 0.5%.

The United Nations Population Fund (UNFPA) blasted the UK’s cut to its reproductive health agency as “devastating” for women, girls and their families around the world.

It said the Government’s expected contribution to the flagship programme for family planning this year would fall from £154 million to £23 million.

Details of the cut were the latest to emerge, after a leaked memo showed the UK would slash bilateral funding for overseas water, sanitation and hygiene projects by more than 80%.

Asked about the UNFPA cut during a visit to a school in west London, the Prime Minister told reporters: “We’re still spending £10 billion per year on overseas aid, that is a huge amount.

“We are still one of the biggest donors in the world and I think people in this country should be very proud of that.

“But I also think that they will understand, and I know that other countries around the world understand, the particular pressures of the pandemic that mean we have to economise in that way.

“You ask me if we will go back to 0.7% – the Government has always been clear that we will when it is fiscally prudent to do so, when we have the headroom to do so.”

UNFPA said the cut to its programme was a “retreat from agreed commitments made to the programme” last year and that funds would have been used to prevent tens of thousands of maternal and child deaths, millions of unintended pregnancies and millions of unsafe abortions.

The organisation said that, in addition, £12 million was to be cut from the UNFPA’s “core operating funds” and that several country-level agreements were also likely to be affected.

UNFPA executive director Dr Natalia Kanem said the decision by a “longstanding partner and advocate” was one it “deeply regrets”.

“These cuts will be devastating for women and girls and their families across the world,” she said.

“The truth is that when funding stops, women and girls suffer, especially the poor, those living in remote, underserved communities, and those living through humanitarian crises.”

A spokesperson for the Foreign, Commonwealth and Development Office (FCDO) said: “The seismic impact of the pandemic on the UK economy has forced us to take tough but necessary decisions, including temporarily reducing the overall amount we spend on aid.”

The spokesperson added that the department was “working with suppliers and partners on what this means for individual programmes”.

Baroness Sugg, who quit her post in the Foreign, Commonwealth and Development Office late last year over aid cuts, described the Government’s decision as a “double hit” on the world’s poorest.

The Conservative peer said: “This is money which the UK committed to in the UN chamber, a promise made in front of delegates of every country in the world, a signed agreement which we’re walking away from, which is pretty unheard of.”

She told BBC Radio 4’s Today programme it was “unprecedented” for the UN to put out a statement saying the money would have helped prevent so many maternal and child deaths and millions of unintended pregnancies.

Plan International UK condemned the cut as a “shameful decision which will result in the unnecessary deaths of tens of thousands more women and girls during pregnancy and childbirth”, and called on the Government to “urgently change course or forfeit any claims to be a champion of global gender equality”.

Rose Caldwell, the charity’s chief executive, said: “(These) cuts are a betrayal of women and girls around the world.

“We urge the Government to come to its senses and reinstate funding for these vital services.”

Dr Edward Morris, president of the Royal College of Obstetricians and Gynaecologists, branded the cut “an unconscionable attack on the health and well-being of women and girls around the world”.