Thein Sein is visiting the United Kingdom from July 14 to 16.
Despite important changes in Burma over the past two years, many serious human rights problems remain, Human Rights Watch said.
Pledges made by Thein Sein, including those to US President Barack Obama in November 2012, to improve human rights remain partially or completely unfulfilled, including granting full humanitarian access to ethnic conflict areas, releasing all remaining political prisoners, amending abusive laws, and allowing the United Nations High Commissioner for Human Rights to establish offices in the country.
''Prime Minister Cameron should not miss an important opportunity to press Burma's president on justice for crimes against humanity committed against the country's Muslims, the release of remaining political prisoners, or an end to repressive laws,'' said David Mepham, UK director at Human Rights Watch.
''Recent improvements in Burma will continue only as long as world leaders keep up the pressure to bring an end to the many human rights abuses still occurring in the country.''
On April 22, European Union foreign ministers lifted sanctions, including targeted sanctions on the Burmese army and government individuals and entities, leaving only its export ban on arms to Burma.
'The scrapping of targeted sanctions on Burma was premature and surrendered key leverage to improve the country's still dire human rights situation,'' Mepham said.
In 2012, state security forces, local Arakanese political party officials, and Buddhist monks participated in crimes against humanity during a campaign of ethnic cleansing against ethnic Rohingya and other Muslims in western Burma's Arakan State.
More than 140,000 Muslims remain in camps, are denied freedom of movement, and lack adequate shelter, humanitarian aid, and basic services. Thein Sein has not fulfilled his pledge to take ''decisive action to prevent violent attacks against civilians,'' hold accountable perpetrators of abuses, and ''address contentious political dimensions, ranging from resettlement of displaced populations to granting of citizenship.''
Humanitarian aid organisations also remain without full access to conflict areas in other parts of Burma, including Kachin State in the north, where a two-year armed conflict between government forces and Kachin rebels has displaced over 80,000 people, and in eastern Burma, where over 400,000 people are displaced from decades of civil war.
The Burmese government should amend or revoke laws and regulations that discriminate against ethnic minorities, Human Rights Watch said.
These include Burma's 1982 Citizenship Law, which effectively denies Rohingya and other ethnic groups the ability to obtain citizenship, even when their families have lived in Burma for generations.
Thein Sein should publicly repudiate a discriminatory decree that limits Rohingya families to two children each.
Burma has still not issued an invitation to the United Nations High Commissioner for Human Rights, and there has been no significant progress in negotiations with the Ministry of Foreign Affairs to set up such an office. Meanwhile, more than 200 political prisoners are estimated to be in Burma's prisons.
''Prime Minister Cameron needs to insist that Thein Sein cease his foot-dragging on pledges he made to improve human rights,'' Mepham said.
''There's no good reason why the UN should not be able to open an office and monitor the human rights situation across the country.''
Doing business in Burma involves various human rights risks, Human Rights Watch said. These include the lack of rule of law and an independent judiciary, major tensions over the acquisition and use of land, and disregard of community concerns in government-approved projects.
The military's extensive involvement in the economy, use of forced labor, and abusive security practices in business operations heightens concerns. Corruption is pervasive throughout the country.
Thein Sein's government continues to use repressive laws to undermine peaceful protests against projects that impact livelihoods and land.
The authorities violently cracked down on people protesting a copper mining project in northern Burma in November and prosecuted demonstrators peacefully protesting against a natural gas project in Arakan State in April.
Major infrastructure projects and land acquisitions for companies have also generated controversy involving both companies and the Burmese military in land seizures.
The UK government should acknowledge that the political reform process in Burma is very far from complete. Key measures of progress include legal reform, free and fair parliamentary elections in 2015, and amendments to the constitution to remove the Burmese military's constitutional authority over civilian government, including ending the military's authority to appoint 25 percent of the seats in the parliament and to dismiss the parliament and president.
''Failure to press the Burmese government to meet its reform commitments will send precisely the wrong message to Thein Sein, leading him to believe his government is no longer under serious international pressure to follow through on reforms,'' Mepham said.
''David Cameron should ensure that this trip is not a 'victory lap' for the Burmese president but rather a public re-commitment by Burma to meet its human rights obligations.''