The letter, on the eve of an emergency summit of European Union leaders in Brussels, condemned the Libyan authorities use of airpower in attacks on their own people as "unacceptable".
And Mr Cameron and Mr Sarkozy called on the EU to unite in calling for Gaddafi to go, arguing that his regime has "lost any legitimacy it may have once had".
"Since the Libyan people have started to rise against Muammar Gaddafi's brutal regime, the world is witnessing on a daily basis an unacceptable continuation of violence and repression in Libya," wrote the two men in a letter to European Council President Herman van Rumpoy, copied to the other 25 EU leaders.
"Ignoring UN Security Council resolution 1970 demands as well as calls from regional organisations and the whole international community, Gaddafi's regime continues to attack his own people, including with aircraft and helicopters. It is clear to us that the regime has lost any legitimacy it may have once had.
"This deliberate use of military force against civilians is utterly unacceptable. As warned by the Security Council, these acts may amount to crimes against humanity. All those involved in deciding, planning or executing such actions must know that they will be held accountable." (read more)