The international community has a duty to act, Britain said and also added that it will be using its position on the UN Human Rights Council to actively press for an international investigation given the lack of a credible domestic accountability process to date.
Also, The UK lauded the progress which has been made by Sri Lanka in areas like reintegration of child soldiers, demining and reconstruction of conflict affected areas.
However, it said it strongly believes "that, without a credible investigation in to very serious allegations of the past, it will be very difficult for the Sri Lankan people to move forward in the true spirit of lasting peace, reconciliation and unity and for Sri Lanka to reach its true potential."
Britain has earlier issued a warning that in the absence of an independent investigation in sexual crimes carried out by government forces in Lanka, "pressure would mount for an international investigation including from the UK".
UK's foreign minister has supported the High Commissioner for Human Rights' call for an international investigation.
British Minister of State at the Foreign and Commonwealth Office (FCO) Hugo Swire said UK was seriously concerned about the human rights situation in Sri Lanka and the lack of progress on accountability.
"These concerns include the undermining of independent institutions such as the judiciary, a 'significant surge' in attacks on religious minorities, reports of sexual harassment and violations, continuation of impunity, continued militarisation of former conflict areas and the continuing trend of restrictions on freedom of expression and association," Swire said.