"Liu Xiaobo should have been allowed to choose his own medical treatment overseas, which the Chinese authorities repeatedly denied him," Johnson said in a statement.
"This was wrong and I now urge them to lift all restrictions on his widow, Liu Xia," he added.
A prominent Chinese dissident, Liu died in a heavily- guarded hospital today after battling liver cancer.
His wife has been under house arrest since 2010, although she was allowed to visit her husband in hospital.
The Chinese authorities had rebuffed offers for Liu to be treated abroad, warning other countries to stay out of China's internal affairs.
Described by Johnson as a "lifelong campaigner for democracy, human rights and peace", Liu was detained in 2008 after calling for democratic reforms.
He won the Nobel Peace Prize in 2010 and an empty chair marked his absence at the awards ceremony in Oslo, as Liu had by then been sentenced to 11 years in prison for "subversion".
Having been transferred to hospital still under guard, Liu became the first Nobel Peace laureate to die in custody since German pacifist Carl von Ossietzky, who passed away in a hospital while held by the Nazis in 1938.
(This story has not been edited by Business Standard staff and is auto-generated from a syndicated feed.)