Shadow foreign secretary Lisa Nandy said "press freedom is a fundamental right" after the attack destroyed the tower containing offices of The Associated Press and Al-Jazeera.
After journalists and other tenants were safely evacuated, three heavy missiles pounded the 12-storey building, razing it to the ground, amid spiralling violence between Israel and Palestine.
The strike came hours after at least 10 Palestinians, mostly children and from an extended family, were killed in an Israeli air raid on a densely populated refugee camp.
The AP news agency said the targeting of the media building is "an incredibly disturbing development" and warned the "world will know less about what is happening in Gaza because of what happened today".
The Israeli military said the building contained military assets belonging to Hamas, the Palestinian militant group that governs Gaza.
Ms Nandy condemned the airstrike and called for the Government to join allies in calling for a ceasefire, amid concerns the conflict could escalate into all-out war.
"The targeting of media offices in Gaza by Israeli airstrikes is completely unacceptable. Press freedom is a fundamental right," she said in a statement.
"The devastating escalation of violence, including Hamas rocket attacks on Tel Aviv and air strikes on the Gaza City refugee camp, has cost more civilian lives and we condemn it in the strongest possible terms.
"The UK must join our international partners in calling for an immediate ceasefire.”
The Foreign Office has not commented on the attack.
Reporting by PA