Sir Keir Starmer was forced to delegate his response to the Budget to his shadow chancellor on Wednesday after testing positive for Covid.
The Labour leader's spokesman announced that he would begin isolating, while Rachel Reeves was sent to respond to Rishi Sunak in the Commons Chamber and Ed Miliband faced Boris Johnson at Prime Minister's Questions.
In a video posted online, Sir Keir said he was "absolutely gutted" to miss the Budget and accused Mr Sunak of doing "nothing" to tackle the rising cost of living.
"I'm fine, but it is obviously important we all follow the rules," he said. "But wasn't Ed Miliband at PMQs and Rachel Reeves in the Budget response just brilliant?
"Both of them stepped up at short notice – very short notice – to absolutely call out the Government for their failure on the climate crisis and to hammer the Chancellor on his smoke and mirrors Budget which does nothing for working people and nothing about the cost of living crisis.
"I'll now be working from home but if you have symptoms or any cause for concern, please get tested to stop the spread of this virus."
Sir Keir has already had to isolate four times after coming into contact with people who have tested positive for Covid. The most recent isolation, in July, was prompted by one of his children testing positive.
It is understood Wednesday's positive test result came just as he was leaving for the Commons. A photo posted online showed him preparing with Ms Reeves and Bridget Phillipson, the shadow chief secretary to the Treasury, before the result came through. Both have since tested negative for Covid.
Mr Miliband said representing Labour at PMQs was "just like old times" after his five-year stint at the despatch box while leading the party between 2010 and 2015.
The former leader was greeted by ironic cheers from Tory MPs when he stood up in the Commons chamber and said he wished to reassure both sides of the House it was for "one time only".
Sir Keir’s deputy leader, Angela Rayner, is currently on bereavement leave after losing a close loved one, her spokesman said on Wednesday.minutes before he was expected to appear at the House of Commons in preparation to respond to the Budget.