Beaten by Francois Hollande in the May 2012 election, Sarkozy has since concentrated on the international conference circuit. However, he has been reportedly considering another tilt at the presidency in 2017 to dethrone socialist Francois Hollande.
Cameron, a conservative like Sarkozy, became prime minister in 2010. The British leader's spokesman denied that the private invitation to a former head of state was unusual.
"They worked very closely together during Mr Sarkozy's time in office," the spokesman told reporters.
"During that time, they established a very strong working relationship. It is not unusual for prime ministers to meet former heads of government who they have worked very closely with."
Asked whether the meeting reflected an attempt by Sarkozy to revive his political profile, the spokesman said the meeting was private.
"The only thing it reflects is the nature of the relationship which they struck up while they were working together. It is a private meeting."
Sarkozy is in London to address a client conference being held in private by US investment bank Goldman Sachs.
At home, however, the former president was in March placed under formal investigation on suspicion of taking advantage of France's richest woman, Liliane Bettencourt, who suffers from dementia, to secure up to 4 million in financing for his 2007 campaign.
He is also to be investigated over allegations that he accepted cash from former Libyan dictator Moamer Kadhafi to fund the 2007 campaign.
Cameron and Sarkozy developed a particular personal bond in 2010 after the French leader provided a helicopter that allowed the British premier to fly to France and say a final goodbye to his dying father.