A civil engineer has said he has "personal knowledge" of five additional counties where properties are affected by defective blocks.

Aidan O'Connell, who is a structural expert, told RTÉ's This Week programme he believes that up to 12 counties in total may be impacted by the issue, and that the total eventual cost of the State's Defective Concrete Blocks Grant Scheme could reach €5 billion.

Last November, the Government unveiled plans to support homeowners in counties Donegal and Mayo affected by mica.

On Friday, an expert group examining the Enhanced Defective Concrete Blocks Grant Scheme said it "appears inevitable" that it will have to be extended to other counties.

Mica is a naturally occurring mineral found in building blocks. Too much mica, however, causes the blocks to absorb water, which ultimately causes them to crumble.

According to the Mica Action Group, blocks containing more than 1% mica may appear good quality on production and can even pass compression tests, but buildings containing these blocks are still very likely to start to crumble within a number of years.

Work is under way to determine if homes in Clare, Sligo, Limerick, and Tipperary should be included along with those in Donegal and Mayo.

However, Mr O'Connell, who has a particular expertise in pyrite and mica-related issues, said he has "personal knowledge" of at least five more counties where properties have issues with defective blocks: Meath, Louth, Dublin, Carlow, and Wexford.

Mr O'Connell also said he has been told the issue is present in houses in Co Offaly.

He said while mica is the predominant issue for houses affected in Donegal, in the other counties pyrite is the main problem.

Mr O'Connell added that although the current number in additional counties where houses are impacted by defective blocks is low, he expects the number detected could eventually be "far in excess of the number of cases we are seeing in Mayo and Donegal".

"I am worried about this increasing over the coming years, because it takes a long time for the pyrite problem to manifest itself in blocks ... it can be anything from ten to 15 years," he said.

The civil engineer says all of the new cases he has seen have been in domestic dwellings.

Mr O'Connell believes the remediation work to address the issues will not be resolved for five-to-ten years "if we're only talking about Donegal and Mayo".

He also said he can see the cost of the State's grant scheme "certainly going well above" the €2.2bn allocated.

"We're also in a different economic environment now, building material prices have gone through the roof, availability of building materials has collapsed, and there is a shortage of qualified construction professionals to carry out the work."

If the scheme is extended to other counties, Mr O'Connell said he expects to see "the bottom line going to €4bn or €5bn at the very least".