In a midnight telephone call, Corbyn told the Leeds Central MP he was kicking him out of the top team.
The Labour spokesman said: "Jeremy has sacked him on the grounds that he has lost confidence in him."
The "plot" emerged after defiant Corbyn vowed yesterday to fight off any challenges to his leadership, telling a crowd of supporters that he would run again if his opponents forced a contest.
It is not the first time the two have clashed, with Benn backing Conservative-led plans for airstrikes against ISIS in Syria against Corbyn's position.
Benn served in the Cabinet under Tony Blair and Gordon Brown.
He is a fourth-generation MP and his father Tony served in Parliament for 47 years, and was a Cabinet minister himself in the governments of Harold Wilson and James Callaghan.
Benn said: "It has now become clear that there is widespread concern among Labour MPs and in the Shadow Cabinet about Jeremy Corbyn's leadership of our Party.
"In particular, there is no confidence in our ability to win the next election, which may come much sooner than expected, if Jeremy continues as Leader.
"At this critical time for our country, following the result of the EU referendum, we need strong and effective leadership of the Labour Party that is capable of winning public support so that we can stand up for the people of Britain.
"In a phone call to Jeremy, I told him that for these reasons I had lost confidence in his ability to lead the Party and he then dismissed me from the Shadow Cabinet.
"I thanked him for having given me the opportunity to serve him and the Party as Shadow Foreign Secretary."
Initial reaction was divided, with some MPs supporting Benn and claiming there were "no good reasons" to stay in the Shadow Cabinet.
Wes Streeting, MP for Ilford North, said on Twitter: "Lots of good people chose to serve in Shadow Cabinet to keep the show on the road. There are no longer good reasons for good people to stay."
Exeter Labour MP Ben Bradshaw added: "The Labour Shadow Cabinet must now act to save the Party and for the sake of the country. Otherwise we will never be forgiven."
Roberta Blackman-Woods, the shadow minister for housing said the sacking was "sad news indeed" and said questioned Corbyn's judgement.
While Liberal Democrat MP Alistair Carmichael said the Labour strife would distract from the chaos at the top of the Conservative party.
Others on social media said "good riddance" and praised Corbyn for "showing some teeth".
Amid growing anger against Corbyn's allegedly "lacklustre" performance for Remain in the EU referendum, there were also claims last night that up to 200 Labour MPs may back the no confidence motion - which could be decided in a secret ballot to be held on Tuesday.
Last night, senior Labour MP Margaret Hodge, who proposed the no-confidence vote, said that although it would not be binding on Corbyn, he should quit if it passed.
"If the motion is carried, I hope he does the decent thing and resigns."
