Heartbroken relatives of British hostages held by Hamas have warned that Keir Starmer is leaving those still held captive to "rot" in dungeons.
Four families declared that the Prime Minister's plan to recognise a Palestinian state will fail to secure peace.
And Ayelet Svatitzky, whose mother was taken hostage after her two brothers were killed, told Foreign Office diplomats "this is a prize for Hamas for the murder of my two brothers".
The families on Friday urged Sir Keir to make the recognition of a Palestinian state conditional on the release of the hostages.
They warned the Prime Minister's current plan "could even hurt" those still held by Hamas terrorists.
In an emotional statement, their lawyers declared after a meeting on Thursday night: "The families held out some hope that the policy could not be as they feared, and that the UK had chosen to impose conditions on recognition, those conditions would also be on Hamas, as otherwise they would essentially be rewarded for continuing to commit war crimes, including hostage taking, and encouraged to continue that path.
"It was on this point that we hoped to obtain clarification.
"It was clear from the meeting that the British government's policy will not help the hostages, and could even hurt them.
"We do not say this lightly, but it was made obvious to us at the meeting that although the conditions for recognising a Palestinian state would be assessed 'in the round' in late-September, in deciding whether to go ahead with recognition, the release or otherwise of the hostages would play no part in those considerations.
"In other words, the 'vision for peace' which the UK is pursuing, and which the families heard much about, may well involve our clients' family members continuing to rot in Hamas dungeons.
"As Ayelet Svatitzky, whose two British citizen brothers Nadav (a hostage murdered in captivity) and Roi Popplewell (who was murdered on 7 October) and whose mother Chana Peri was taken hostage, said at the meeting: 'This is a prize for Hamas for the murder of my two brothers".
Sir Keir on Tuesday said the UK would recognise Palestine before the UN General Assembly in September unless Israel agreed to certain conditions, including taking steps to end the humanitarian crisis in Gaza and committing to a ceasefire.
The Prime Minister defended his plan, insisting "we do need to do everything we can to alleviate the humanitarian catastrophe in Gaza, where we are seeing the children and babies starving for want of aid which could be delivered."
Lawyers Adam Rose and Adam Wagner, speaking on behalf of British hostage families, added: "665 days on from October 7 2023, the families' priority is to keep the hostages above political games.
"The time that has passed has only served to make the situation for those held even worse.
"We implore all parties involved to remember the immense suffering of the hostages. Every snatched video of hostages serves as another arrow in their hearts.
"The British hostage families implore the Prime Minister to change course before it is too late."
It comes after Labour faced fresh fury from the White House over their stance on Gaza.
US Secretary of State Marco Rubio branded the UK's position 'so clumsy' as Hamas will not agree to a ceasefire if they already know a Palestinian state will be recognised.
Speaking to Fox News Radio, Mr Rubio said: "The UK is like, well, 'if Israel doesn't agree to a ceasefire by September, we're going to recognise a Palestinian state'.
"So if I'm Hamas, I say, 'you know what, let's not allow there to be a ceasefire.' If Hamas refuses to agree to a ceasefire, it guarantees a Palestinian state will be recognised by all these countries in September.
"So they're not going to agree to a ceasefire. I mean, it's so clumsy."
Mr Rubio suggested Sir Keir was under pressure from the Labour Party and pro-Gaza critics of the government.
He said: "What this really is, in many of these countries' case, is their domestic politics. Some of these countries have huge constituencies now that are pressuring them domestically to line up on this side, irrespective of its geopolitical ramifications."
It came after White House press secretary Karoline Leavitt told reporters on Thursday: "The president expressed his displeasure and his disagreement with the leaders of France, the United Kingdom and Canada.
"He feels as though that's rewarding Hamas at a time where Hamas is the true impediment to a ceasefire and to the release of all of the hostages."
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