David Cameron and Vladimir Putin are to hold talks on the Syrian civil war.
The British Prime Minister and Russian President will discuss the crisis during a bilateral summit in Downing Street.
Mr Cameron is likely to raise Russia's continual blocking of a United Nations Security Council resolution designed to increase the pressure on President Bashar Assad as he presses on with his brutal crackdown on pro-democracy fighters in a conflict which has led to at least 19,000 deaths.
The high-level talks at No 10 come two weeks after Russia and China vetoed the latest UN attempt to end the bloodshed, and as Syrian government forces continue their battle to regain full control of Syria's biggest city, Aleppo.
Russia's backing for President Assad has meant repeated efforts for a new UN resolution have failed.
UN relief co-ordinator Baroness Amos claimed that the lack of a resolution to establish a safe zone in Syria was hampering attempts to help civilians caught up in fighting in the Middle Eastern state. Lady Amos said a resolution would be needed to establish a secure corridor for the delivery of aid supplies or a safe zone for people fleeing violence in the 17-month uprising.
Describing the conditions in Aleppo and other parts of Syria as "extremely worrying", the UN's under-secretary-general for humanitarian affairs said the overriding need was for an effective ceasefire. But President Assad hailed his army's military action against what he termed "terrorist criminal gangs", saying their battle will determine the fate of the country.
Meanwhile, Mr Cameron was also under pressure to raise the 2006 death of Kremlin critic Alexander Litvinenko, who was allegedly poisoned with Polonium 210 by a Russian agent, with President Putin.
Mr Litvinenko's widow, Marina, condemned the Russian leader's London visit, saying she was "dismayed". She added: "To show that Mr Putin is not welcome here, I urge my fellow Londoners to put on a white ribbon, the symbol of Russian protest, on the day Mr Putin arrives."
Judo black belt President Putin will reportedly use his visit to the city, his first trip to the capital for nine years, to watch the martial art, his favourite sport, at the Olympic Games.