British Labour Party leader Keir Starmer has tested positive for Covid-19.
House of Commons speaker Lindsay Hoyle said Mr Starmer would be replaced by his party's business spokesman Ed Miliband at Prime Minister's Questions in the House of Commons because he was isolating.
Yesterday, the House of Commons ordered everyone except MPs will have to wear face coverings while on the parliamentary estate due to rising Covid-19 cases.
However, officials said Mr Hoyle would be encouraging elected representatives, despite not being obliged to wear a face covering, to put one on in the Commons chamber unless they are speaking or are exempt.
The ruling comes after MPs were urged to wear masks during today's Budget speech by a coronavirus expert from the World Health Organization.
Dr David Nabarro, the WHO's special envoy for Covid-19, said that "everybody" should be wearing masks in close confinement with other people, "including our leaders".
Health Secretary Sajid Javid has already committed to wearing a mask in the Commons when it is packed on Budget day, having urged politicians to lead by example following a recent spike in cases.
But Mr Javid said on Monday that wearing a mask in the crowded chamber is a "personal decision" for ministers and backbenchers.
Commons Leader Jacob Rees-Mogg last week insisted Tories do not need to wear masks in Parliament because with their "convivial, fraternal spirit" they know each other well.
Chancellor Rishi Sunak and Justice Secretary Dominic Raab were among those Conservative MPs wearing masks in the Commons chamber, although Mr Rees-Mogg was not.
Mask-wearing on the Tory benches was mixed during Prime Minister's Questions.