The civil service's biggest union has warned the government that it is "seriously considering" taking legal action to allow departmental officials to cease involvement in the export of arms to Israel.
PCS is understood to be seeking an urgent meeting with senior officials at the Department for Business and Trade over concerns that the supply of UK weapons to Israel is illegal under international law because of its conduct in Gaza.
The move comes days after seven aid workers were killed in an Israeli strike in Gaza. Three of the victims were UK nationals.
The United Nations Office for the Co-ordination of Humanitarian Affairs said this week that "at least" 32,975 Palestinians had been killed in Gaza since October 7 last year. The escalation of the conflict was prompted by a murderous incursion into Israel masterminded by militant group and Gaza-governing organisation Hamas that left 1,200 people dead and saw a further 250 kidnapped.
This week's aid-convoy attack caused former national security adviser Lord Peter Ricketts to call for the UK to stop selling arms to Israel.
"I think the time has come to send that signal," he told BBC Radio 4's Today programme. "I think there's abundant evidence now that Israel hasn't been taking enough care to fulfil its obligations on the safety of civilians."
Ricketts said halting arms sales would send a "powerful political message" to Israel's prime minister Benjamin Netanyahu that could spark debate about the United States following suit.
PCS head of bargaining Paul O'Connor said the union was contemplating legal action to protect members because the UK government seemed unwilling to take reasonable steps to halt the conflict in Gaza.
"The International Court of Justice considers that some of the alleged acts by Israel in Gaza could potentially be considered within the provisions of the Genocide Convention," he said.
"PCS concurs with that view. We believe that the UK government has an obligation to do all it can to halt the onslaught. As it does not appear to be willing to do so, we are seriously considering taking legal action to prevent our members from being forced to carry out unlawful acts.
"We do not take such cases lightly and we only do so where we have reasonable prospect of winning."
Dave Penman, general secretary of civil service leaders' union the FDA, called on the government to set out the advice it has been given on the legality of arms exports to Israel.
He pointed out that the civil service code requires officials to "uphold the rule of law".
Penman told talk-radio station LBC that while civil servants were well-aware that their job was to act on the instructions of ministers of a democratically-elected government, the situation was "different" when questions of legality came into play.
"There are serious figures who've been in government, in the diplomatic world, and former national security advisers who are raising these concerns about whether the government is now in a position [where] it's breaking its own rules around the sale of arms, where there's a legitimate concern they're going to be used to break international humanitarian law," he said.
"I think we're in the grounds of where this is a legitimate question to be asked and the government really need to settle it. They need to get on with this.
"The foreign secretary said nearly a month ago that they were getting a definitive legal position. They're backtracking on that now. And I think what everyone needs to know is what is the settled government view on the legal position."
Speaking on condition of anonymity, a senior government lawyer told Civil Service World that there was a difference between work that is "high risk" and work that is "illegal" and that significantly more information would be required for a conclusion to be drawn.
"In the meanwhile, the more junior civil servants have been tasked to get on with something which they've been told is lawful, that's been subject to a judicial review, and that's a reasonable management instruction," they said.
"It is not for junior members of staff to opine whether something is legal or not, it's for the lawyers and the ministers."
A government spokesperson said: "We keep advice on Israel’s adherence to international humanitarian law under review and will act in accordance with that advice.
"All export licence applications are assessed on a case-by-case basis against the strategic export licensing criteria."