Britain's national postal service has marked the 100th birthday of a former soldier who has raised nearly 30 million pounds ($A58 million) to help Britain's health service through the coronavirus pandemic.

The Royal Mail will stamp a special postmark onto all letters sent between Monday and Thursday, when ex-army captain Tom Moore turns 100.

Hundreds of thousands of people donated to Moore's JustGiving fund for National Health Service (NHS) charities after he walked 100 25-metre lengths of his garden over several days, supported by his walking frame.

"The postmark will pay tribute to the work of the inspiring NHS fundraiser who has captured the hearts and minds of the nation in the midst of the ongoing crisis," the Royal Mail tweeted.

In his first speech since returning to work on Monday following his infection with coronavirus, Prime Minister Boris Johnson said Moore's "spirit of optimism and energy" could inspire Britain as it tries to to curb the spread of Covid-19.

The Royal Mail said last week that it had handled more than 40,000 birthday cards, letters and gifts for "Captain Tom."

Moore also reached the top of Britain's singles charts last week, joining singer and actor Michael Ball and an NHS choir on a version of the 1960s hit "You'll Never Walk Alone."

The underfunded NHS has struggled to cope with thousands of critically ill people infected with Covid-19, amid a shortage of protective equipment for its staff.