In her resignation letter to the prime minister, David Cameron, Warsi wrote that Britain's support for Israeli military action against Hamas, which has resulted in the deaths of more than 1,800 Palestinians over the past month, "is morally indefensible, is not in Britain's national interest and will have a long term detrimental impact on our reputation".
Warsi's decision to quit makes hers the first ministerial resignation 'on principle' since the coalition was formed in 2010 and comes in the wake of attacks on the prime minister's handling of the Gaza crisis by Labour leader Ed Miliband - and a 72-hour humanitarian truce agreed between Israel and Hamas in Cairo on Monday evening.
Speaking exclusively to The Huffington Post UK in her first interview since resigning on Tuesday morning, the Tory peer accused the coalition of failing to act as an "honest broker" in the Middle East and called for an immediate arms embargo against Israel.
"The British government can only play a constructive role in solving the Middle East crisis if it is an honest broker," the former Foreign Office minister said, "and at the moment I do not think it is."
Warsi was appointed by Cameron as chair of the Conservative Party and minister without portfolio in May 2010 - becoming the first Muslim to serve as a full cabinet minister. Last week, however, Channel 4 News reported that FCO officials believed Warsi had "deep reservations and concerns about government policy" on Gaza.
The Tory peer told HuffPost UK that one of the reasons she resigned on Tuesday morning, despite the signing of yet another temporary ceasefire agreement between Israel and Hamas, was because she wanted to see those who are alleged to have committed war crimes over the past four weeks, both in Gaza and in Israel, held to account - but did not believe that the British government would support that process. "As the minister for the International Criminal Court, I’ve spent the last two and a half years helping to promote, support and fund the ICC. I felt I could not reconcile this with our continued pressure on the Palestinian leadership not to turn to the ICC to seek justice."
The former Conservative Party chair, who was moved to the post of Senior Minister of State at the Foreign and Commonwealth Office in the September 2012 reshuffle and permitted to continue attending Cabinet, revealed in her interview with HuffPost UK that she had been struggling with the coalition's stance on the Israel-Palestine conflict since November 2012. "Our position not to recognize Palestinian statehood at the UN in November 2012 placed us on the wrong side of history and is something I deeply regret not speaking out against at the time."
Now that she has quit the government, the Tory peer wants to "speak more freely" on this issue and her first demand after handing in her resignation letter is for the UK to introduce an arms embargo. "It appalls me that the British government continues to allow the sale of weapons to a country, Israel, that has killed almost 2,000 people, including hundreds of kids, in the past four weeks alone. The arms exports to Israel must stop."
Her resignation will put further pressure on the prime minister to take a harder line against Israel's bombing and invasion of the Gaza Strip and follows interventions from an array of leading Conservative politicians who have expressed unease over mounting civilian casualties on the Palestinian side, including former Tory defence ministers Nicholas Soames and Peter Luff and influential backbencher Margot James, parliamentary private secretary (PPS) to former foreign secretary William Hague.
"I ask that the government rethinks policy towards the conflict in Israel and the Palestinian territories," wrote James in a (leaked) letter to Philip Hammond, Hague's successor at the Foreign Office, last week.
Warsi told HuffPost UK that she was a "long-standing supporter of Conservative Friends of Israel, and someone who fundamentally believes in Israel right to to exist and be secure" but explained that she "couldn’t sit silently by as the Israeli military committed acts that have been described by [UN Secretary General] Ban Ki Moon as 'moral outrages' and 'criminal acts' and by the French foreign minister as 'massacres'".
The peer said she was left frustrated by the UK's reluctance to strongly condemn Israel's repeated killing of civilians in Gaza - in contrast to other national governments and international bodies across the world. "The UN is 'morally outraged', the US government has called the shelling of schools as 'totally indefensible'. Meanwhile, the British government has been dragging it heels."
The former Foreign Office minister made clear she was not defending Hamas attacks on Israel. "Hamas is a terrorist organization," she told HuffPost UK, "and there can never be any excuse for it to fire rockets targeting civilians in Israel. It too must be held accountable for the misery it inflicts upon both the Israelis and the Palestinians."
However, she added, "Israel as an occupying power has a responsibility of course to protect Israelis but it also has a responsibility to protect Palestinians".
In her resignation letter, which she posted on Twitter, Warsi told the PM that her decision had not been "easy" and while he continued to have her "personal support" she had to be "able to live with myself for the decisions I took or the decisions I supported. By staying in Government at this time I do not feel I can be sure of that."
Speaking on LBC radio, London mayor Boris Johnson described Warsi's decision to stand down over Gaza as "very sad" and said he had "great respect" for her.
Here is the text of Baroness Warsi's letter of resignation in full:
Dear Prime Minister
For some weeks, in meetings and discussions, I have been open and honest about my views on the conflict in Gaza and our response to it.
My view has been that our policy in relation to the Middle East Peace Process generally but more recently our approach and language during the current crisis in Gaza is morally indefensible, is not in Britain's national interest and will have a long term detrimental impact on our reputation internationally and domestically.
Particularly as the Minister with responsibility for the United Nations, The International Criminal Court and Human Rights I believe our approach in relation to the current conflict is neither consistent with our values, specifically our commitment to the rule of law and our long history of support for International Justice. In many ways the absence of the experience and expertise of colleagues like Ken Clarke and Dominic Grieve has over the last few weeks become very apparent.
The decision has not been easy. It has been a privilege to serve for 3 years in your Shadow Cabinet and over 4 years in your Cabinet. Introducing you in Blackpool in 2005 as you made your bid for leadership I had the pleasure of being there at the start of the journey and it would have been rewarding to have been there till the end.
The last decade has given me the opportunity to work with some of the best in the Conservative Party and indeed in government. William Hague was probably one of the finest Foreign Secretaries this country has seen and has been inspirational. He dismantled foreign policy making by sofa government and restored decision making and dignity to the Foreign Office. There is however great unease across the Foreign Office amongst both Ministers and senior officials, in the way recent decisions are being made.
Eric Pickles has supported me tirelessly in our work on combating hate crime, challenging anti-Semitism and Islamaphobia and the pioneering work of celebrating faith in the public sphere. This new found confidence in Government has allowed me to take the very public international lead on religious freedom, specifically on the ever growing crisis of the persecution of Christians.
However, early evidence from the Home Office and others shows that the fallout of the current conflict and the potential for the crisis in Gaza and our response to it becoming a basis for radicalisation could have consequences for us for years to come.
From both Eric and William I learnt the art of reconciling passion and idealism with pragmatism and realism, but I always said that long after life in politics I must be able to live with myself for the decisions I took or the decisions I supported. By staying in Government at this time I do not feel I can be sure of that.
It is therefore with regret that I am writing to resign.
You will continue to have my personal support as leader of the Conservative Party as you continue to ensure that our Party evolves to meet the challenges we face in Britain today and ensure that the Party is relevant and responsive to all communities that make up today's Britiain.