The report into spoiled election ballots at the Holyrood election in May is to be delayed by two months because of the weight of evidence being considered.

Ron Gould, the Canadian elections expert conducting the review, yesterday said interviews had taken twice as long as planned and his team had not yet been able to start examining the 180,000 ballot forms that had not been counted because voters had failed to fill them in properly.

The fiasco meant several constituencies were won by a smaller margin than the number of spoiled votes. Thousands of postal ballots were delivered too late and a new electronic counting system failed to work properly, leading to the result being delayed for hours.

Mr Gould was meeting journalists in the final stages of taking evidence and said the report is now due for publication on the week of October 21.

A change in the law has been required at Westminster to let his team see more than 140,000 spoiled forms in the Holyrood ballot. The Scottish Executive has given permission for him to see spoiled forms for the council elections, for which Holyrood has responsibility.

It is only now that the team will be able to start the task of assessing them.

He stressed his inquiry was independent of the Electoral Commission.

This follows concerns that the commission, which asked him to carry out the investigation, was liable to criticism for its role in planning the election.

But he said, apart from his appointment and funding, he has been left free to expand his remit and to ask questions of anyone: "We have been very comfortable we have been able to take this review anywhere we have wanted to take it."

Mr Gould has been involved in elections in Canada for 20 years and an adviser on them in South Africa, Cambodia, Kosovo and the electoral problems in the Florida count at the 2000 US presidential election.

He said that his report would offer options rather than recommendations, so politicians and election administrators could decide which were appropriate in Scotland.

The most contentious issue could be around the design of the Holyrood ballot, which was unlike the 1999 and 2003 papers. At the top of the left column, most voters faced the option of "Alex Salmond for First Minister" instead of the SNP name, and that confusion is thought to have worked substantially in the SNP's favour.

Since the election, it has become clear that although the redesigned ballot form was tested on the public, the survey was not extensive and the paper tested was not the same one used in the election. The survey results showed the same level of confusion as seen in the election.