Finance minister Edwina Hart has reacted angrily to last week's email to assembly members from Lord Rogers. In it, the architect denied that his practice had allowed cost overruns amounting to about £11m over the original construction budget, which was the reason cited for sacking it from the project in July.
In a confidential letter to assembly members, Hart accused RRP of more than 100 underestimates of the cost, scope and content of its work in the original cost plan.
She added that a report from Symonds, the assembly's QS, identified potential subcontractor preliminary costs that RRP had failed to cover.
Hart's nine-page letter expands on her brief rejoinder last Wednesday to RRP's email. She said she had been prompted to go into greater detail by members' concerns about issues raised by RRP.
She said: "I had not wanted to be drawn into a public dispute with Lord Rogers about blame, but his allegations have made it necessary to set the record straight."
Hart said she exchanged 18 letters with Rogers between 10 May and 7 September. In one of these, she asked him whether he could build the assembly within the £13.8m construction budget.
"His response indicated only that he thought he could deliver the building for less than £24m and offered no explanation of how savings could be achieved."
Hart added that Rogers' claims that he had warned the assembly about the cost implications of using Welsh materials and the inadequacy of contingency provisions were evidence that the practice could not bring the project in on budget.
She said: "It begs the question why RRP chose to submit a cost plan that they apparently already thought was likely to prove deficient."
RRP said it would be inappropriate to comment on a confidential letter.
Despite the spat, Mike German, a member of the committee that decided Rogers' fate, said he hoped RRP would retender to show that there were "no hard feelings".
The deadline for expressions of interest to take the project forward was this Wednesday. Until last week, fewer than five developers and architects had applied.