A London patient continued to test positive for Covid-19 for 505 days, the longest on record.
It is feared the person who tested positive for two years caught a variant which caused persistent infection before they died.
The previous longest known PCR confirmed case is thought to be 335 days.
British researchers have also found one of the first occult Covid infections.
Occult infection means the patient was thought to have cleared the virus, with negative testing to prove it but the infection was ongoing but was undetected.
British researchers studied nine individuals in London who had weakened immune systems due to organ transplantation, HIV, cancer or medical therapies for other illnesses.
These nine individuals tested positive for the virus for at least eight weeks.
Infections persisted for 73 days, on average, but two patients had persistent infections for more than a year.
Five of the nine patients have survived.
While one of the patients, who had Covid-19 for 505 days, died, there is another patient who has had an ongoing infection for 412 days.
They are being treated with monoclonal antibodies to try to clear their infection.
If they continue to test positive, it is likely they will pass the 505-day record mark.
Dr Luke Blagdon Snell, of Guy’s and St Thomas’ NHS Foundation Trust, said: “New variants of SARS-CoV-2, the virus that causes COVID-19, have emerged throughout the pandemic.
“Some of these variants transmit more easily between people, cause more severe disease, or make the vaccines less effective.
“One theory is that these viral variants evolve in individuals whose immune systems are weakened from illness or medical treatments like chemotherapy, who can have persistent infection with SARS-CoV-2.
“We wanted to investigate which mutations arise, and if variants evolve, in these people with persistent infection.”
“This provides evidence that mutations found in variants of concern do arise in immunocompromised patients and so supports the idea that new variants of the viruses may develop in immunocompromised individuals.
“It is important to note, however, that none of the individuals in our work developed new variants that became widespread variants of concern.
“Additionally, whilst this work shows variants could arise in immunocompromised individuals, whether the previous variants of concern like Alpha, Delta and Omicron arose in this manner remains unknown.”
Dr Gaia Nebbia, co-author, says: “Immunocompromised patients with persistent infection have poor outcomes, and new treatment strategies are urgently needed to clear their infection. This may also prevent the emergence of variants.
“Occult infection describes someone who is thought to have cleared the virus, for instance with negative tests, but is later found to have an ongoing infection.
“This has been described with other viruses such as those that cause Ebola or hepatitis B and is different to long Covid where the virus is generally thought to be cleared from the body but symptoms persist.
“The patient was symptomatic and tested positive for Covid before recovering.
“They then tested negative several times before developing Covid symptoms again several months later.
“A PCR test was positive and genome sequencing of the virus at this point showed the infection was caused by the Alpha variant, which had by then been eliminated from the UK, suggesting the virus had been present in the body ever since the initial infection but remained undetected.”
The research was presented at the European Congress of Clinical Microbiology & Infectious Diseases (ECCMID) in Lisbon, Portugal.