More than 8,000 Bosnian Muslims were murdered during the atrocity in the town of Srebrenica. The United Nations described it as "the worst crime on European soil since the Second World War".
David Cameron will host a reception at 10 Downing Street after the service. He's set to announce that the government will spend an extra £1.2m on the commemoration of the 1995 massacre.
Mr Cameron said: "The 20th anniversary is a moment to remember the many thousands who lost their lives, to remember their families and the missing, and the fact that for so many - including the Mothers of Srebrenica - the agony continues every day, undimmed by the passage of time.
"We must reaffirm our determination to act to prevent genocide in the future."
About 100 events have been organised across the UK as part of Srebrenica Memorial Week. First Minister of Wales Carwyn Jones will host a commemorative event on Wednesday in Cardiff and Nicola Sturgeon will host a memorial service at St Giles' Cathedral in Edinburgh on Friday.
The people of Srebrenica were supposed to be in a 'UN safe haven' when the Bosnian Serb army invaded. Thousands of men and boys were slaughtered, then buried in mass graves.
Ratko Mladic, 72 was the Bosnian Serb general in charge of the troops responsible for the genocide. He was arrested in 2011 and is now on trial at The Hague, charged with crimes against humanity, murder and genocide. Mladic has denounced the court as "satanic".
Srebrenica will told its own memorial on 11 July, the day the Serbs took the town. Commemorations will include the burial of 136 recently identified victims.
1,400 men and boys who were slaughtered in the massacre are still yet to be found or named.
It is estimated that as many as 50,000 women were raped during the Bosnian war.
The president of the Serb-run part of Bosnia claimed on Saturday that the Srebrenica genocide was a "lie". He also denounced the British attempt to have the events at Srebrenica recognised as genocide within a UN security council resolution. It is reported that Serbia has asked Russia to veto the resolution.
