Sub-postmasters faced legal battles and even prison sentences after being accused by fraud - a claim they deny - while using the point-of-sale system which they believe caused a discrepancy between cash and sales.
Following pressure from a campaign led by MP James Arbuthnot, the Post Office commissioned forensic accountants Second Sight to investigate the claims.
The report released in September found some aspects of Horizon was "unfit for purpose".
Since, the postal firm opened up for negotiations with those accused, but today MPs accused the Post Office of barring certain sub-postmasters and "doing it in secret".
Speaking on Radio 4 this morning, Rt Hon James Arbuthnot said he had "no confidence that the Post Office is trying to clear it up" and accused the firm of blacklisting the majority of sub-postmasters from the negotiation process.
Mark Davies, Post Office's communications director said that the firm was "really sorry if people have faced lifestyle problems."
However, he added that the Horizon system is used "every single day by 80,000 people" that have not reported issues.
Davies would not say how many of the accused had been barred from the mediation process.
The Second Sight report found that preliminary investigators for the Post Office accused sub-postmasters of theft or false accounting instead of finding the root cause of mismatched figures and that equipment was outdated.