He added that it was not an emergency admission and that the prime minister was quote "doing well".
"News that Prime Minister Boris Johnson was admitted to hospital saw the British pound dip, only to recover quickly since it seems it was just precautionary", said Jasper Lawler, head of research at London Capital Group.
Foreign secretary Dominic Raab, seen as de facto deputy prime minister, is expected to take Johnson's seat in chairing the government's daily coronavirus meeting on Monday.
Johnson, 55, was admitted to hospital Sunday night for routine testing, but remained there on Monday.
"Stay home, protect the NHS and save lives", he tweeted.
He has continued to preside at daily meetings on the outbreak and has released several video messages during his isolation.
"The reasons some people get seriously ill with COVID-19 while others have minor symptoms is not yet fully understood, ?"
He had been scheduled to re-emerge on Friday after a week of recovery and working remotely, but said he would remain at home because he still had a high temperature - one of the symptoms.
"I feel better", Johnson said in a video posted on his Twitter account.
The prime minister said he had experienced mild symptoms over the past 24 hours, including a temperature and cough, but would continue to lead the British government. "Together we will beat this".
The UK has reported over 48,000 cases of COVID-19.
"As of 5pm on April 5, of those hospitalized in the United Kingdom who tested positive for coronavirus 5,373 have died", the United Kingdom health ministry said.
US President Donald Trump was among the well-wishers, starting Sunday night's White House press briefing by saying Mr Johnson was "a great friend of mine" and adding: "I'm sure he is going to be fine, he's a strong man, a strong person".