The UK will send surveillance aircraft and two Royal Navy ships to the eastern Mediterranean in plans “to support Israel”, No 10 says.
The aircraft will begin patrols on Friday to “track threats to regional stability such as the transfer of weapons to terrorist groups”.
The package includes surveillance assets, helicopters, P8 aircraft, and a company of marines.
Rishi Sunak said the military support would “prevent further escalation”.
The British armed forces will be on standby to “deliver practical support to Israel and partners in the region, and offer deterrence and assurance”, Downing Street said.
Under the plans, a Royal Navy task group will be moved to the area next week to support humanitarian efforts.
The prime minister said the government “must be unequivocal” in making sure the “horrific scenes” that have unfolded in Israel “will not be repeated”.
“Our military and diplomatic teams across the region will also support international partners to re-establish security and ensure humanitarian aid reaches the thousands of innocent victims of this barbaric attack from Hamas terrorists,” Mr Sunak said.
He has also asked for military teams in Israel, Cyprus and across the region to be bolstered to support contingency planning.
Hamas launched an unprecedented attack on southern Israel on Saturday, killing at least 1,300 and taking around 150 hostages to Gaza.
More than 1,300 have also been killed in Gaza since Israel launched retaliatory air strikes.
Defence Secretary Grant Shapps said the military aid “will be an undeniable display of the UK’s resolve to ensure Hamas’s terrorist campaign fails”.
Meanwhile, the Foreign Office has begun arranging flights for British nationals stranded in Israel.
The first plane is expected to leave Tel Aviv on Thursday, with more planned “in the coming days, subject to security”.