Mr Woolfe, 49, was rushed to hospital after the “scuffle” at the EU Parliament in Strasbourg, France, earlier this month.
The former barrister alleged Mike Hookem MEP had “punched him” — a claim Mr Hookem denies.
But Mr Woolfe has today suddenly quit as UKIP MEP.
The politician, who was born in Moss Side, Manchester, abandoned his bid to become leader of the party, claiming it was “ungovernable” without Nigel Farage leading it, and the European Union referendum cause to unite behind.
Mr Woolfe said he will continue sitting as North West MEP as an independent.
His announcement came after UKIP said its new leader would be announced on November 28.
Mr Woolfe said: “The events at the meeting in Strasbourg led to me later being treated by doctors for two seizures, partial paralysis and loss of feeling in my face and body.
“I would like to thank all of the medical staff in Strasbourg for their impressive and swift treatment. I owe them my life. I would also like to thank my family and friends for their love and support — and also all of the people in politics who have sent me their best wishes.
“Since then, I have spent some time considering my personal and political future.
“It is with deep sorrow and regret that I am aborting my leadership campaign and announcing my resignation from Ukip with immediate effect.”
He added: “With regards to the highly-regrettable events in Strasbourg, I will reiterate my position that I received a blow from Mr Hookem that knocked me back into the meeting room and caused my subsequent injuries. Contrary to reports, I have made a police complaint.
“I am now seeking legal advice in respect of the investigations and will not be commenting further on the matter until the completion of those investigations.”