UK Prime Minister David Cameron has condemned the blockade of the Gaza Strip, describing the territory as a "prison camp".
He also criticised Israel for launching an attack on a convoy transporting Turkish activists and aid to Gaza. Nine Turkish citizens died in the raid.
He was speaking to an audience of businessmen during a visit to Ankara.
The Israeli embassy in London said Gazans were prisoners of Palestinian militant Islamist group Hamas.
Israel and Egypt enforce a blockade on Gaza which restricts goods and people from coming in or out freely.
"Gaza cannot and must not be allowed to remain a prison camp," Mr Cameron said.
"People in Gaza are living under constant attacks and pressure in an open-air prison," he said.
In May, Israeli commandoes stormed the Mavi Marmara and in fighting that followed, nine Turkish activists were killed and four soldiers wounded.
During a press conference held with Turkish Prime Minister Recep Tayyip Erdogan, Mr Erdogan called the Israeli raid an act of "piracy".
"Israel must apologise as soon as possible, pay compensation and lift the blockade," he said.
The British government's policy has been to call for an end to the blockade, but never before has a British prime minister been so blunt, says the BBC's Jonny Dymond in Ankara.
A spokesman for the Israeli embassy in London said Hamas, which won Palestinian parliamentary elections in 2006, was responsible for the situation in Gaza.
"The people of Gaza are the prisoners of the terrorist organisation Hamas. The situation in Gaza is the direct result of Hamas' rule and priorities," the spokesman said.