Mr Cameron's comments came during a visit to Turkey, where relations with Israel have been strained since Israeli troops stormed a flotilla of ships carrying supplies to Gaza in May, killing eight Turks and one Turkish-American.
Speaking in Ankara, the Prime Minister denounced the attack on the flotilla as ''completely unacceptable'' and restated his call for Israeli PM Benjamin Netanyahu to deliver a ''swift, transparent and rigorous'' inquiry.
But he also urged Turkey not to allow the incident to wreck its relationship with Israel.
Israel's blockade of the Gaza Strip, which has severely limited the movement of people and goods since 2007, has sparked outrage in Turkey, which provided the organisers and the bulk of the participants for the flotilla.
Today Mr Cameron said: ''The situation in Gaza has to change. Humanitarian goods and people must flow in both directions.
''Gaza cannot and must not be allowed to remain a prison camp.''
And he added: ''The Israeli attack on the Gaza flotilla was completely unacceptable.
''And I have told PM Netanyahu we will expect the Israeli inquiry to be swift, transparent and rigorous.''
The Prime Minister said he hoped that direct talks between Israel and the Palestinians would take place within the coming weeks and urged Turkey to press the parties to come together by ''making the case for peace''.
He said: ''Turkey's relationships in the region, both with Israel and with the Arab world, are of incalculable value.
''No other country has the same potential to build understanding between Israel and the Arab world.
''I know that Gaza has led to real strains in Turkey's relationship with Israel.
''But Turkey is a friend of Israel.
''And I urge Turkey, and Israel, not to give up on that friendship.''
At a press conference alongside Turkish Prime Minister Recep Tayyip Erdogan, Mr Cameron stood by his comments on Gaza, which he said were "warranted" by the situation there.
Mr Cameron added: "I speak as someone who is a friend of Israel, who desperately wants a secure and safe and stable Israel after the two-state solution has come about.
"It is very important that people remember that Israel will only agree to the final status issues if it feels that at the end of that process it will have the security that it craves.
"That is why on the issue of Gaza, while pushing for humanitarian access and the end of the blockade, we always have to remember that there have been rocket attacks from Gaza on Israel."
Mr Erdogan said Gaza resembled "a sort of open-air prison" and described the Israeli army assault on the aid flotilla as "piracy".
"The fact that this blockade has not been lifted is a tragedy," said the Turkish PM.
Addressing the raid on the flotilla, he said: "This attack in international waters can only be termed piracy. There is no other word to describe it.
"I hope we can remedy this situation and Israel turns back from this mistake. They must apologise to Turkey and compensation has to be paid and the blockade must be lifted so we can all contribute to regional peace."