Sir Keir Starmer was confronted on a train to Glasgow by a man calling for him to back a ceasefire in Gaza.
The Labour leader has come under pressure over his stance on the ongoing conflict in the Middle East as the picture continues to get bleaker for Palestinians enclaved by Israeli forces.
UN secretary-general Antonio Guterres used a rarely exercised power on Wednesday, urging members of the UN Security Council to demand an immediate humanitarian ceasefire as the conflict in Gaza continues.
His letter to the council’s 15 members said Gaza’s humanitarian system was at risk of collapse after two months of war that has created “appalling human suffering, physical destruction and collective trauma”.
He demanded civilians be spared greater harm as he invoked Article 99 of the UN Charter.
Mr Guterres said: “The international community has a responsibility to use all its influence to prevent further escalation and end this crisis.”
A short draft resolution was circulated to council members late on Wednesday by the United Arab Emirates, the Arab representative on the council.
The resolution demands “an immediate humanitarian ceasefire” and expresses “grave concern over the catastrophic situation in the Gaza Strip and the suffering of the Palestinian civilian population”.
Starmer has indicated that his steadfast stance on refusing to back calls for a ceasefire in Gaza is about showing his credentials to be the next prime minister.
He said that “working with our international allies” to free hostages taken by Hamas and push for pauses in the fighting so Palestinians can find safety is “what you would expect from someone who wants to form the next government”.
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