"The Prime Minister is not only my colleague and my boss but also my friend and my thoughts are with him and his family," he added.
Earlier, Johnson's spokesman said that he was "responding to treatment" while being kept at hospital for close monitoring" and remained clinically stable.
The disease has struck at the heart of the British government, infected more than 60,000 people across nationwide and killed over 7,000.
Johnson was admitted to St Thomas' on Sunday, on the advice of his doctor, after continuing to have a cough and high temperature 10 days after testing positive for the virus. He was given oxygen before being taken to intensive care on Monday.