The army's announcement appeared to signal a long-threatened Israeli ground invasion is imminent.

Shadow foreign secretary David Lammy called for an "immediate ceasefire" and said an Israeli offensive in Rafah "would be catastrophic".

Israel says Rafah is the last significant Hamas stronghold but had previously paused plans to attack the city in southern Gaza so hostage release negotiations could take place.

However, Israeli defence minister Yoav Gallant claimed on Sunday that Hamas was not serious about a deal and the army was preparing "a powerful operation in the very near future in Rafah".

On Sunday, Hamas set off rockets from Rafah towards Kerem Shalom, Israel's main crossing point for delivering aid, killing three Israeli soldiers.

Overnight, Israeli strikes killed at least 19 people, including a baby, according to Palestinian health officials.

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Israel's army has told about 100,000 people to evacuate eastern Rafah to a humanitarian zone designated by Israel on the Mediterranean coast.

Rafah, Gaza's most southern city, on the Egyptian border, is where more than a million people - more than half Gaza's population - have taken refuge during the war that began last October.

Joining other Western nations and humanitarian organisations in urging Israel not to strike Rafah, Sir Keir said on social media: "With more than a million Palestinian civilians sheltering in Rafah, an Israeli offensive must not go ahead.

"There must be an immediate ceasefire, the immediate release of all hostages, and unimpeded aid into Gaza that can be delivered regularly, quickly and safely."

Read more: Why has Israel's offensive prompted widespread international condemnation?

Mr Lammy wrote: "An Israeli offensive in Rafah would be catastrophic. It must not go ahead.

"We need an immediate ceasefire, the immediate release of hostages, and immediate unimpeded aid to Gaza."

UK Foreign Secretary Lord Cameron earlier said the UK is "very concerned" about the situation in Rafah and called for Israel to "stop and think seriously" before taking further action.

Charity ActionAid said forcing Palestinians from Rafah "without a safe destination is not only unlawful but would lead to catastrophic consequences".

They said "there are no safe zones in Gaza" and aid workers have seen some of the "most severe conditions in recent memory" with widespread disease, starvation and chaos.

Madeleine McGovern from Care International UK said ministers need to urgently suspend licences for arms sales to Israel to prevent an expansion of military operations in Rafah.

"It would be unconscionable for British-made weapons to be used in an assault on Rafah," she said.

Islamic Relief warned the area where Palestinians have been ordered to move, al Mawasi, is not safe and that forcing more people there will make the humanitarian crisis worse.