Multiple failings meant the Met Police failed to catch electoral fraud during the 2014 Tower Hamlets mayoral election, according to a watchdog.
Former mayor Luftur Rahman was found guilty of corrupt and illegal practices by an Election Court in 2015.
An "unco-ordinated" police operation meant potential leads were disregarded before the election, HM Inspectorate of Constabulary found.
Scotland Yard said: "We welcome the findings."
Cdr Stuart Cundy said: "We will carefully consider whether further action is needed in order to prevent failings in the future.
"Since 2014 we have taken significant steps to improve how we communicate with our partners and local communities, which includes a robust system for reporting allegations of electoral fraud and receiving feedback."
The 2014 Mayoral election in Tower Hamlets was overturned following a six-week High Court hearing found evidence of vote-rigging and malpractice.
The court action was the result of a private action brought by four citizens.
Lutfur Rahman was subsequently removed from office, but has not faced prosecution.
Her Majesty's Inspectorate of Constabulary and Fire & Rescue Services (HMICFRS) found a lack of oversight from senior officers led to a number of failings into policing around the overturned election.
Potential lines of enquiry were disregarded, while an "unco-ordinated approach" to investigations meant that opportunities might have been missed, HMICFRS said.
The Met's 2017 re-investigation of the election known as Operation Lynemouth was "painstaking, thorough and achieved its objectives", according to HMICFRS.
"Upholding the integrity of our democratic processes is crucial if the public are to have full confidence in elections," Sophie Linden, Deputy Mayor for Policing, said.
Ms Linden, who commissioned the report, said "it's clear the original investigation in 2014 was not up to the standards we would expect and it's disappointing that there was a need for a new investigation, which came at a sizeable cost to the London taxpayer".