About 115,000 postal staff were striking on Friday in an ongoing dispute over pay.
It is the first of four days of action, with walkouts also taking place on August 31 and September 8 and 9.
Letters will not be delivered during the walkout and Royal Mail warned that many parcels will also be delayed.
The UK has already seen a summer of strikes from barristers and port, rail and communications workers, while nurses, doctors and civil servants have also threatened industrial action.
Communication Workers Union general secretary Dave Ward warned that staff across multiple industries were considering strikes at the same time as the cost-of-living crisis grows.
He said: “The CWU’s also at the centre of a campaign called Enough Is Enough and there will be, I think, at some point, forms of collective action taking place across the UK.
“The way that companies in the UK, particularly companies like Royal Mail, are working at the moment is just not sustainable.
“They’ve got to start looking after the people who made the very profits that have got them in a good position.”
Royal Mail said it has offered workers up to 5.5 per cent in salary increases.
But the CWU is demanding a rise more closely in line with inflation, currently at 10.1 per cent and forecast to hit 18 per cent early next year.
It comes as energy prices are set to increase by 80 per cent in October and as food costs continue to soar.
Mr Ward added: “This is the biggest strike in the UK since 2009, and we have got over 115,000 members out on strike today after delivering a massive ‘yes’ vote — 97.7 per cent... the company made record profits last year: £758 million.
“They gave away over £400 million to shareholders, they rewarded themselves with huge record bonuses for achieving their financial targets... it’s simply not acceptable.”
But a Royal Mail spokesman said the union wants to “cling to outdated working practices.”
It comes as rail unions threaten to continue walkouts, with drivers on the Croydon Tramlink set to strike on September 12.
US senator Bernie Sanders is due to join an RMT rally next Wednesday as the union continues to fight with TfL and the Government over pay and pensions.
Workers at the Port of Felixstowe also began eight days of industrial action this week.
Tory frontrunner Liz Truss has promised new laws to make it harder to call strikes and to guarantee minimum services on public transport.