Sadiq Khan and Labour MPs have piled the pressure on Sir Keir Starmer to call for an immediate ceasefire in Gaza.
The official Labour party line is that there should be a “pause” in the conflict to allow aid in and hostages to get out of the besieged Palestinian territory.
But the Mayor of London went further on Friday and said there needed to be a stop to the fighting between Israel and Hamas to prevent further killing and a “protracted conflict in the region” that would lead to “further devastating loss of life.
“Many Londoners like me have watched the absolute horror at the death and suffering in Israel and Gaza,” Mr Khan said.
“I join the international community in calling for a ceasefire. It will stop the killing and would allow vital aid supplies to reach those who need it in Gaza.”
He was supported by several Labour MPs, including Dawn Butler and Marsha de Cordova.
Brent Central MP Ms Butler said: “I agree with [Mr Khan’s] statement. An immediate ceasefire is necessary and a freeing of all hostages.”
Scottish Labour leader Anas Sarwar also joined called for a ceasefire, saying "there have been too many innocent lives lost in Israel and Palestine."
Nine Arab countries, including key US allies and nations that have signed peace or normalisation deals with Israel, have issued a joint statement calling for an immediate ceasefire, a demand set to be discussed at an emergency meeting of the United Nations’ General Assembly.
“The right to self defence by the United Nations Charter does not justify blatant violations of humanitarian and international law,” said the statement, signed by Egypt, Jordan, Bahrain, The United Arab Emirates, Saudi Arabia, Oman, Qatar, Kuwait and Morocco.
In London, a Cabinet minister said Britain will not ask Israel to “put down arms” in a ceasefire.
Education Secretary Gillian Keegan said the UK instead backed a “pause” in the fighting to allow desperately-needed aid into Gaza.
It comes after Sir Keir faced a wave of criticism from Muslim MPs and councillors over his stance on the conflict.
In an LBC interview, three weeks ago, the Labour leader said “Israel does have that right” to cut off power and water to Gaza. He later clarified he does not support Israel committing war crimes.
But the party has not yet called for a full ceasefire.
Sir Keir and his deputy Angela Rayner met with Muslim MPs and peers earlier this week in a bid to calm tensions.
Around a third of Labour MPs have now reportedly said they either want a ceasefire or signed a Commons motion calling for one.
Dozens of councillors have resigned the whip, including several in London.
One London Labour councillor told the Standard on Friday: “I don’t know if it’s too late now for Keir to do something that will stop the anger.”
Shadow environment secretary Steve Reed said he understands Labour colleagues who disagree with the party’s position.
But he added Israel “have the right under international law” to defend themselves.
Asked if Labour's stance on will have an electoral impact, Mr Reed said: "I think (it) won't because and the reason I think that is in politics, you should do the right thing, not the electorally expedient thing."
A Labour Party spokesperson said: "Of course, we understand why people want to call for a ceasefire. The Palestinian people are not Hamas, and they are suffering terribly. That’s why we support humanitarian pauses so that aid, fuel, water electricity and medicines can urgently get to those who need it.
“We also have to recognise Israel was subject to a vile terrorist attack. Israel has a right and a duty to defend itself, rescue the hostages and stop Hamas from being able to carry out that sort of terrorist attack ever again.
“Hamas are currently firing rockets into Israel and have built the infrastructure, including tunnels, from which to carry out further attacks, so that military operation is ongoing. That must be done within international law and aid must get in quickly, safely, and regularly to halt a humanitarian disaster."
