Lutfur Rahman - directly-elected mayor of Tower Hamlets in east London - was found “personally” guilty of wrongdoing and “guilty by his agents” by Election Commissioner Richard Mawrey.
Mr Mawrey said the 2014 Tower Hamlets mayoral election was void and that Mr Rahman could not stand at a new election.
Four voters took legal action against Mr Rahman at a hearing in the High Court in London which ran for several weeks earlier this year.
The group was headed by Andy Erlam, who had stood as a councillor on an anti-corruption ticket.
“It is a fantastic result for democracy,” he said after today’s ruling.
“There will have to be a new election for mayor. Mr Rahman cannot stand.”
The four voters mounted a challenge under the provisions of the Representation Of The People Act.
Lawyers for the group made a series of allegations, including “personation’’ in postal voting and at polling stations, and ballot paper tampering.
Mr Rahman had said there was “little, if any” evidence of wrongdoing against him.
His lawyers described the group of four’s claims as invention, exaggeration and “in some cases downright deliberately false allegations”.
But Mr Mawrey made a series of findings against Mr Rahman - who was born in Bangladesh in 1965.
He said the effect of his ruling was that “Mr Rahman’s election as mayor on 22 May 2014 was void - that is to say, it is as if it had never taken place. He has not lawfully been mayor since that date”.
He said “a new mayoral election” would have to be arranged.
He added: “It is declared that Mr Rahman shall be incapable of being elected to fill the vacancy for the office of Mayor of the London Borough of Tower Hamlets.”
“The evidence laid before this court ... has disclosed an alarming state of affairs in Tower Hamlets,” he said.
“This is not the consequence of the racial and religious mix of the population, nor is it linked to any ascertainable pattern of social or other deprivation. It is the result of the ruthless ambition of one man.”
He added: “The real losers in this case are the citizens of Tower Hamlets.”
Mr Rahman was not in court to hear Mr Mawrey’s verdict.
Sources estimated the total cost of the litigation to be in the region of £1 million.
The judge ordered Mr Rahman to make a £250,000 payment on account of the costs incurred by the four voters and other parties involved.
Barrister Duncan Penny QC, who represented Mr Rahman, challenged the sum claimed, describing it as “mind-boggling”.
Mr Mawrey said: “If the money isn’t there, you will have to fight it out in an insolvency.”
The group of four voters had said there were “serious questions’’ which “need answers’’.
Barrister Francis Hoar, who represented the group, told Mr Mawrey: “The allegations against Lutfur Rahman are that he was guilty of corrupt and illegal practices, directly or through his agents.’’
Mr Hoar said his clients were accusing Mr Rahman of “election fraud” and that there had been “personation’’ in postal voting and at polling stations.
He said people had registered themselves or others to vote at addresses at which they did not live.
And he said there had been tampering with ballot papers.
Mr Hoar said Mr Rahman was also accused of “making false statements” about the “personal character’’ of his main rival - Labour candidate John Biggs.
He alleged “undue influence’’ by “means of spiritual influence’’ during the election campaign and on polling day.
He complained that canvassers had been paid.
He alleged “undue influence through intimidation at polling stations’’ and “interference with voters’’ - including in polling booths.
He alleged “bribery’’ through “unlawfully diverting public funds to organisations in order corruptly to procure their political support’’.
He said those organisations included religious organisations and ‘media organisations’.
Mr Hoar said there had either been “particular corrupt acts’’ perpetrated by Mr Rahman or his agents or “general corruption’’, which “so extensively prevailed that it may be reasonably be supposed to have affected the result’’.
He said Mr Rahman should be found “guilty of corruption and illegal practices’’.
Mr Hoar said the “sulphur’’ had “long been bubbling’’ in the London Borough of Tower Hamlets.
He said people had pointed to the “spectre of corruption’’ and “election fraud’’ but had been “ignored’’ and “called racist’’.
Mr Hoar said witnesses had “reported threats of violence’’.
The judge made a series of findings in the voters’ favour.
A council spokesman said: “Today, the Election Commissioner, Judge Richard Mawrey, found in favour of the arguments presented by the petitioners in this case.
“Mr Mawrey also presented a judgment which cleared the council’s returning officer, John Williams, and council staff of all allegations related to fraudulent practice in the delivery and administration of the 2014 elections.
“We welcome recognition that the council’s strong electoral processes - which have been subject to further intense scrutiny during this petition hearing - are sound and Tower Hamlets Council will now take the steps necessary to hold an election for executive mayor of Tower Hamlets.”