The extra sampling is done as Johnson has continued symptoms of the disease ten days after he tested positive for the coronavirus.
"On his doctor's advice, he has been enrolled in hospitals for tests," Johnson's spokesperson said in a statement on Sunday night.
"It's a precaution," the spokesman continues, adding that Johnson took the opportunity to thank the British healthcare system for its efforts during the corona crisis when he was received at the hospital.
His office says the measure is not of an acute nature, the BBC writes.
Johnson continues to lead government work despite the hospital stay, but Foreign Minister Dominic Raab will chair a government meeting on the virus outbreak on Monday morning, according to the BBC.
Boris Johnson's partner, Carrie Symonds, has written on Twitter that she too has symptoms that are consistent with covid-19, but that she has not tested.