David Cameron on Friday became the first British prime minister to set foot in Myanmar since its independence in 1948.
After his arrival in the capital Naypyidaw, Cameron said: "This country really matters. For decades it has suffered under a brutal dictatorship. It is also desperately poor. It doesn't have to be this way."
"There is a government now that says it is committed to reform, that has started to take steps, and I think it is right to encourage those steps."
Cameron was scheduled to meet President Thein Sein and pro-democracy leader and Nobel laureate Aung San Suu Kyi, The Sun reported.
Cameron described Suu Kyi as "a shining example for people who yearn for freedom, for democracy, for progress".
"We should be under no illusion about what a long way there is to go."
He said the Myanmar government had to show that the move towards democracy was "irreversible".
Developments in the country may be "one potential chapter of light in a world where there are many dark chapters in history being written", he said.
"Of course we should be sceptical. Of course we should be questioning. Of course we shouldn't be naive. Aung San Suu Kyi herself, who has spent so many years in such a long, lonely but powerful struggle, believes that he (Thein Sein) is acting in good faith," Cameron said.