Nobel laureate Malala Yousafzai has urged Britain not to cut overseas aid, ahead of a major announcement by the country's finance minister on Wednesday.

Rishi Sunak is expected to suspend a legal commitment to spend 0.7 percent of gross national income on international development.

Reports suggest he will cut the level of aid to 0.5 percent in his Spending Review, as the government seeks to support the coronavirus-ravaged economy and looks for savings from an aid budget worth £15 billion ($20 billion, €17 billion).

In a tweet late on Tuesday, Yousafzai reminded Sunak and Prime Minister Boris Johnson of the 0.7 percent pledge, renewed in last year's Conservative election manifesto. "When you announce spending priorities... I hope you'll deliver on that promise," she wrote.

The Pakistani education campaigner wrote that leaders must "prioritise education" as "Covid-19 could force 20 million more girls out of school". Her plea came as five former prime ministers also opposed the planned cut.

Among them was former Conservative leader John Major, quoted by The Times newspaper on Wednesday as saying the spending cut was "morally wrong and politically unwise".

Former Labour prime minister Tony Blair said on Saturday that Britain's overseas aid budget had an impact "measured literally in millions of lives".

David Cameron, whose coalition government enshrined the 0.7 percent in law, has said abandoning it would be a "moral, strategic and political mistake".

Johnson's government has repeatedly committed to maintaining the spending and his Conservatives made it a key plank of the election manifesto last year. The government also promised not to grab ring-fenced aid money when it merged the foreign and development ministries earlier this year.

But Sunak on Sunday told Sky News the UK was under "enormous pressure and stress" and faced an "economic shock". Any cut -- even a temporary one -- is likely to trigger a battle with Conservative MPs.

Tom Tugendhat, who chairs parliament's Foreign Affairs Committee, wrote in The Times: "If we cut aid we'll fall behind." He retweeted Yousafzai's message, saying the UK needs "others to join us" as other G7 nations spend less on foreign aid.

The leaders of 187 charities including Save the Children, Greenpeace UK and Christian Aid on Friday urged Johnson not to cut the aid budget.