Microsoft's antivirus software, Microsoft Security Essentials, has failed a certification test by German antivirus research firm AV.
The tests, which were carried out over September and October, challenged home user security products against real-world scenarios, giving each a score of between zero and six in three separate categories: protection, repair and usability.
Out of 18 available points, Microsoft Security Essentials scored 10.5, falling short of the 11 needed for certification to be awarded.
The software performed particularly badly in the protection category, scoring just 1.5 out of six points.
Within this section of the evaluation, the product scored less than 70 per cent for protection against zero-day malware attacks, compared to an industry average of 89 per cent.
Of the 24 security products put to the test, Microsoft Security Essentials was the only one to fail, although many others scored lower than in the previous round of testing carried out in May and June.
Top of the charts was Bitdefender Internet Security, which was awarded 17 out of 18 points.