Image caption Dame Helen Mirren picked up her award last month

Three judges have resigned from the Evening Standard Theatre awards after it emerged Dame Helen Mirren won a prize following a voting alteration.

Dame Helen, who won best actress as the Queen in The Audience, was lagging behind at the end of a secret ballot.

Standard critic Henry Hitchings went on to change his vote, so two actresses no longer tied in first place.

The paper's editor and awards judge Sarah Sands said the move was "an absolutely legitimate choice".

"It would be absolutely wrong to suggest that there was anything untoward about the process," she added.

A judge confirmed to the BBC that the other five panellists were unaware that any changes had been made.

It is not known which of the other shortlisted actresses - Linda Bassett, Lesley Manville, Billie Piper and Kristin Scott Thomas - originally had the same number of votes and tied with Dame Helen at the top.

Three of the judges from the seven-strong panel - theatre critics Charles Spencer, Georgina Brown and Susannah Clapp - will not return for next year's 60th anniversary.

One of them said that "one of the awards seemed like a fix".

Ms Sands said that Mr Hitchings transferred his original vote as he had been a supporter of actress Rosalie Craig, who was subsequently moved into the best musical performance category.

His second choice - Dame Helen Mirren - became his first choice, making her the outright category winner. Ms Sands' vote remained unchanged.

Image caption Rosalie Craig (centre) was moved to a new category, resulting in a vote transfer

The move was "a perfectly legitimate and long-standing element of judging", according to an Evening Standard spokesman.

"One judge's first choice in the category in question was a winner in a different category, therefore his second choice became his first vote. The result of the transferred vote, without a clear winner in the category, gave us our best actress," he added.

Charles Spencer, writing in The Telegraph, said his "jaw dropped" when Dame Helen was awarded the prize, and this was the "one glaring exception" which didn't strike some of the judges as fair.

"My impression from the discussions was that the strongest support for best actress was for Linda Bassett for her beautiful and very funny performance in Arnold Wesker's Roots at the Donmar, and Lesley Manville for her superb and harrowing portrayal of Mrs Alving in Ibsen's Ghosts at the Almeida."

Mr Spencer added they were told to bear in mind the Evening Standard's readership when making their choices.

"In the end, awards are no more than the icing on the theatrical cake, but this year it left a bitter taste in the mouth," he concluded.

Critic Libby Purves, who will remain on the panel, said: "There was a raising of eyebrows at one of the awards," but added that other results were satisfactory to the judges.

Ms Sands said that, for the 2014 ceremony, the system would be made open rather than secret.

Daily Mail critic Baz Bamigboye and Paul Taylor from The Independent are to become judges on next year's panel.