Greg Brown, who was appointed last year as an independent "facilitator" of discussions, said he saw little prospect of the Scottish Parent Teacher Council and the Scottish School Board Association (which recently rechristened itself the Scottish Parent Councils Association) working together in a single body.
But both bodies have attacked his report, Issues and Options for the Formation of a National Parent Body in Scotland, which is published today. The SPTC described it as "a flawed report coming out of a flawed process", while the SPCA said they "expected as much from a consultant appointed by the former Scottish Executive".
For his part, Mr Brown argues: "Any evolution of either of the existing national bodies into a single national body for the future would entail a radical change from the scale and character of their current operation, and is a poorly supported option.
"It was not possible to bring SPTC and SSBA together during this facilitation to explore areas of consensus or difference on the shape and character of a national parent body for the future."
Instead, he proposes the creation of a steering group of up to 25 members to settle the details of the role, constitution and initial financing of an emergent new body.
Mr Brown also warns that taking on an advisory role to individual parents would be a considerable undertaking.
A national body might require a capacity "significantly in excess of that residing in the two current national bodies", and need 12 staff and annual running costs of pound;600,000, he suggests.
Judith Gillespie, development manager of the SPTC, said her organisation had withdrawn from Mr Brown's "facilitation" process because they felt it was "irrevocably flawed". She also said he had spelt out the role of a national body as "promoting parental involvement in children's learning", which she described as "surely only a small step removed from advocating home education for all".
Donald Gunn MacDonald, vice-president of the new Scottish Parent Councils Association, the former SSBA, insisted his organisation had the potential to be a national body representing parents from each local authority in Scotland. "There is nothing complicated in this," he said.
"It is the right of each parent council and area parent council forum to have a voice at a national level. There is no need for a complex overlay of personnel to allow this to happen."
John Stodter, general secretary of the Association of Directors of Education in Scotland, described the report as "logical and clear".
"We want something comprehensive and inclusive - maybe the best way to do that is to start afresh," he said.
A spokeswoman for the Scottish Government said it planned to continue the post of National Parental Involvement Co-ordinator, based in Learning and Teaching Scotland, until 2010; establish a new field team to work with parents; and provide local authorities with extra funding to support parental representation.