- Patel was summoned by PM May to answer questions on unsanctioned meetings
- After the meeting, PM May's office released Patel's letter of resignation
- It is not clear who would replace Patel as British aid minister
Patel, a Brexit campaigner who is popular in the ruling Conservative Party, had to abandon a trip to Africa earlier on Wednesday after being summoned by May to answer questions on more unsanctioned meetings that breached diplomatic protocol.
After a hastily arranged meeting not long after Patel landed in London, May's office released her minister's letter of resignation, in which Patel said her conduct in Israel had fallen "below the high standards" required of her post.
"While my actions were meant with the best of intentions, my actions also fell below the standards of transparency and openness that I have promoted and advocated," Patel wrote in the letter to May.
"I offer a fulsome apology to you and to the government for what has happened and offer my resignation."
May responded in a letter, saying: "Now that further details have come to light, it is right that you have decided to resign and adhere to the high standards of transparency and openness that you have advocated."
It was not clear who would replace Patel, who held the meetings during a holiday in Israel earlier this year.
Under British protocol, a cabinet minister would normally organise meetings through the foreign office and be accompanied by officials, and visits with Israelis would typically be balanced with meetings with Palestinians.
It is the second resignation in May's top team in a week, underlining her weakness at a time when she faces the complicated task of unravelling more than 40 years of ties with the European Union and holding a deeply divided party together.
Patel's meetings with Israeli officials, which May's office said they were not aware of, and a reported visit to an Israeli army field hospital in the Golan Heights, have increased the pressure on the prime minister, who depends on a Northern Irish party in parliament to pass legislation.
But while acting quickly to force Patel to resign, the loss of an enthusiastic Brexit campaigner from her cabinet team of top ministers could mean that May faces greater pressure from lawmakers to pursue a clean break from the EU.
"Priti is more popular than Theresa, and has more contacts and influence overseas," a senior Conservative source said.
"If I was May, I wouldn't want Priti unleashed on the back benches," the source said, using a term to describe lawmakers.
Patel was sacked a week after May's ally Michael Fallon, her defence secretary, resigned in a sexual harassment scandal that was also caught up two other ministers, including her deputy, Damian Green.
In a week of crises, her foreign minister, Boris Johnson, on Tuesday was pressed to apologise over remarks he made about a jailed aid worker in Iran, which critics said could prompt the Islamic Republic to hand her a longer jail term.
With Brexit talks all but hamstrung over how much Britain should pay to leave the European Union, opposition lawmakers and other critics are now openly questioning whether May can continue as prime minister.
"There are times when a government has the stench of death about it," Pat McFadden, a lawmaker from the main opposition Labour Party, told parliament on Tuesday.