The Post Office failed to deliver postal votes in time to be counted and may have skewed the outcome of two elections in the City of London, The Times has learnt.

The delay has raised further doubts over the viability of postal voting before next month’s local elections.

The City of London Corporation is so concerned that postal ballot papers for the mayoral and London Assembly elections on May 1 may not be delivered on time that it is seeking assurances from the Greater London Authority and the Royal Mail about the arrangements.

The concern follows the late delivery of postal votes for two by-elections in the City. Nearly half the postal votes cast in the ward of Farringdon Within were not received in time to be opened and counted in the April 9 poll. Postal votes for Billingsgate also arrived late. Chris Duffield, the town clerk and chief executive of the City of London Corporation, said: “It appears that these votes may have been held by the Post Office for some time.”

More than 600,000 people have applied to vote by post in the mayoral and London Assembly elections. It is estimated that this figure will reach 700,000 once all the applications have been processed and sent out. In 2004, the date of the last election for the London mayoralty, just 423,000 postal votes were issued and more than 250,000 were returned.

Mr Duffield has written to Anthony Mayer, the returning officer for the mayoral and London Assembly elections, expressing his concerns. Meanwhile, the corporation is investigating the delay and Graeme Smith, the losing candidate in Farringdon Within, is preparing to present an election petition in the High Court.

The postal votes for the two by-elections were dealt with by the Mount Pleasant sorting office, one of the largest in London and the one that will deal with tens of thousands of votes in the forthcoming mayoral election. All three parties have been trying to boost the number of postal votes.

A spokesman for the Royal Mail said: “The City of London’s freepost account lapsed in February despite a letter from Royal Mail urging the corporation to renew it but when we were informed only the day before the poll that freepost ballot papers had been sent out, we immediately did everything we could to locate those papers and deliver them.”

The Royal Mail delivers prepaid envelopes even if the customer’s account has run out of money. It recovers the cost at a later date. When questioned by The Times about the votes that arrived after polling day, the Royal Mail could offer no explanation.

Delivering democracy

— Postal voting was first offered in the 1918 general election to soldiers returning from war. It was later made available to the disabled and people too ill to visit polling stations. In 1985 it was extended to people on holiday

— Since 2000 voters no longer have to give a reason for requesting a postal vote. The Government began to encourage postal voting, hoping to increase voting levels

— All-postal voting was tried out in the 2003 local elections. Each area testing it experienced an increase in turnout. Two thirds of voters said that the schemes had made voting easier

— More extensive postal voting in the 2004 local and European elections resulted in several scandals, leading to a new code of conduct

— In the 2005 general election, 12.1 per cent of British voters voted by post, three times the number in 2001

Source: Electoral Commission; Electoral Reform Society; Times archives

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