David Miliband has announced he has quit his role with Sunderland football club following the appointment of controversial new manager Paolo Di Canio.
The former foreign secretary, who is stepping down as an MP, said he had resigned as vice-chairman and non-executive director of the club due to the Italian's "past political statements".
Di Canio, 44, has been accused of fascist views after appearing to give a straight-arm salute to fans while playing for Italian club Lazio.
In a statement released on his website, Miliband said: "I wish Sunderland AFC all success in the future. It is a great institution that does a huge amount for the North East and I wish the team very well over the next vital seven games.
"However, in the light of the new manager's past political statements, I think it right to step down."
Unite Against Fascism's joint national secretary, Weyman Bennett, said the appointment was "an insult" to the people of Sunderland who died fighting fascism.
Bennett said: "If he wants to be a manager and wants to be a public figure, he needs to make it clear he accepts these views are completely inappropriate.
"Football has made great strides in opposing fascism. It's simply not true that you can say you agree with fascism and that's okay. It's something that isn't acceptable."
He said he was "not surprised" by Miliband's decision to step down.
Di Canio has joined Sunderland on a two-and-a-half year deal, after Martin O'Neill was sacked by the struggling Premier League side.
The former West Ham striker has admitted in the past to having fascist leanings, telling Italian news agency ANSA in 2005: "I am a fascist, not a racist."
He has also courted controversy most recently following his acrimonious departure as manager of League One club Swindon in February.
Despite concerns Sunderland chairman Ellis Short said he believed Di Canio is the man to steer the Black Cats away from the relegation zone.
"Paolo is hugely enthused by the challenge that lies ahead of him. He is passionate, driven and raring to get started," Mr Short said in a statement on the club's official website.
"The sole focus of everyone for the next seven games will be to ensure we gain enough points to maintain our top-flight status.
"I think that the chances of that are greatly increased with Paolo joining us."
Miliband joined the Sunderland board in February 2011 having retired from frontline politics after losing the Labour leadership race to his brother Ed.
He revealed last week he was quitting as South Shields MP to take up a role with the International Rescue Committee in New York as he sought to end the "soap opera" surrounding his relationship with his brother.
Di Canio is no stranger to controversy as a player or manager.
In 1998, while playing for Sheffield Wednesday, he pushed referee Paul Alcock to the ground after being sent off in a clash with Arsenal at Hillsborough.
Di Canio received an 11-match ban from the Football Association, including a statutory three matches for being sent off in the previous month's game against Arsenal. He was also fined £10,000.
The Italian was then pictured in 2005 making a raised-arm salute to a group of supporters of Lazio, where he was playing.
He was given a one-match ban and fined £7,000 for the incident, and was also banned for a match following a similar incident earlier in 2005.
In 2011, when Di Canio was appointed as Swindon Town's manager, the GMB union withdrew its sponsorship of the club, citing his political views.
However, he received widespread praise while playing for West Ham when he picked up the ball during a match with Everton after the opposition's goalkeeper lay injured.
Di Canio was awarded Fifa's Fair Play Award in 2001 in recognition of his "special act of good sportsmanship".