Problems with London's ballot papers - which contributed to the rejection of hundreds of thousands of votes - were known some time ago, organisers admit.
Many voters put their first and second choices in the same column
Altogether 56,862 votes for mayor and 167,071 London assembly ballot papers were ruled inadmissible.
Deputy Returning Officer John Bennett told the BBC laws to simplify the voting process were not passed in time.
The Electoral Commission has promised to look at all aspects of the elections, including ballot design.
Elsewhere around the country, the lengthy European election postal ballot forms seemed to increase the number of spoilt papers, with almost double the numbers of rejected votes in the four all-postal regions.
Londoners had to register four votes altogether, a first and second choice for mayor, a vote for a London Assembly candidate and one for an Assembly party.
While 2.9% of papers for the mayoral ballot were rejected, the London Assembly paper proved even more difficult.
Over a hundred thousand people, 6.7% of the electorate, failed to correctly fill out the section choosing a constituency member, while 2.53% did not correctly register a London-wide party choice.
Many of the spoilt papers were caused by people putting their first and second choices for mayor in the same column.
The returning officers wanted flexibility over the layout of the ballot paper, a spokeswoman for London Elects - the body which organised the elections - told BBC News Online.
Rejected votes - selected European constituencies
London - 0.94%
Eastern - 0.23%
*East Midlands - 0.20%
*North East - 1.3%
*North West - 0.96%
South East - 0.54%
South West - 0.36%
* All-postal vote constituency
But the legislation to allow them to do this was not passed in time.
Mr Bennett said: "The government only had a particular window of opportunity to change the legislation and we missed that window."
The Electoral Reform Society said it had warned of problems after the mayoral vote in 2000.
It had wanted to see full instructions written on the ballot paper itself, but the government refused, instead publishing fuller instructions on the manifesto booklets.
"Whatever system you decide upon you need proper voter education," the society's Alex Folkes said.
"The instructions should be on the ballot paper - it's the one piece of paper that every voter is guaranteed to see.
"Three hundred thousand people did not see their preference counted.
"We will fight for a proper review as this cannot be allowed to happen again."
The Campaign for Plain English also weighed into the debate, saying the lesson to be learnt was to avoid having two different voting systems on the same paper.
"If we have to have different elections held on the same day, the rule must be a totally separate ballot paper for each election with a clear explanation of the voting system," a spokesman said.
"Ballot papers should be properly tested in simulations before election day.
"When real elections are used for testing ballot designs, democracy pays the price of failure."