Theresa May has accused her successor Boris Johnson of "abandoning" the UK's moral leadership on the world stage.
The ex-prime minister said Mr Johnson's decision to cut the overseas aid budget below 0.7% of national income had reduced the UK's global "credibility".
She wrote in the Daily Mail the UK had to "live up to its values" and would be judged by its actions not its rhetoric.
Mr Johnson said the UK was "embarking on a quite phenomenal year" of global leadership.
Questioned about Mrs May's comments by the SNP's Westminster leader Ian Blackford at Prime Minister's Questions, Mr Johnson said: "I think it's very important the prime minister of the UK has the best possible relationship with the president of the United States.
"That's part of the job description."
Mr Blackford called on the PM to reverse "his cruel policy of cutting international aid for the world's poorest".
Later on Wednesday, Joe Biden will be inaugurated as the 46th president of the United States, succeeding Donald Trump.
In advance of the event, Mr Johnson said he looked forward to working "hand-in-hand" with the new administration and that post-Covid challenges could only be tackled by "international co-operation".
But, in an article in the Daily Mail, Mrs May suggested Mr Johnson had squandered international goodwill by choosing not to meet the longstanding UN target of spending 0.7% of income on international development.
The government says it cannot meet the figure - enshrined in UK law - this year because of the strain placed on the public finances by the pandemic.
Theresa May has made these criticisms - on overseas aid and the threat by the government to override international law - before.
Quite often she gets a dig in when she stands up in the House of Commons.
But packaging it all up in this way, on this day, is, in the words of one of her close former advisers, "quite punchy".
The government would rather focus on the relationship it is going to forge with the new US president.
Ministers feel they have quite a lot in common with Joe Biden when it comes to working together on the world stage, fighting climate change and co-operating on global security.
Mrs May also criticised Mr Johnson's support for legislation which could have allowed the UK to go back on parts of its Withdrawal Agreement with the EU, had it been passed.
Controversial clauses were ultimately removed from the Internal Market Bill in December, after the UK and EU reached an agreement.
But Mr Johnson's threat to break international law was criticised in Europe and the US - where Mr Biden warned it could imperil peace in Northern Ireland.
Mrs May said the UK was "well placed to play a decisive role in shaping this more co-operative world but to lead we must live up to our values".
"Other countries listen to what we say not simply because of who we are, but because of what we do. The world does not owe us a prominent place on its stage," she added.
"Whatever the rhetoric we deploy, it is our actions which count. So, we should do nothing which signals a retreat from our global commitments."
Mrs May, who had a sometimes strained relationship with Mr Trump, said Mr Biden's election presented the UK with a "golden opportunity" for Western democracies to reverse the trend towards "absolutism" - and a "few strongmen facing off against each other" - in global affairs.
The UK holds the presidency of the G7 this year and hosts the COP26 climate summit in Glasgow.
Mr Johnson said he looked forward to welcoming Mr Biden to the UK at least twice in 2021.
"In our fight against Covid and across climate change, defence, security, and in promoting and defending democracy, our goals are the same and our nations will work hand-in-hand to achieve them," he added.