Mr Shields appeared before Bulgaria's Supreme Court in Sofia yesterday to appeal against his conviction.
The court did not consider any new evidence, but whether Mr Shields' original trial was flawed on procedural grounds. If it finds the original trial was flawed, the Supreme Court can order a retrial.
Mr Shields' father,, also called Michael, attended court to support his son and said they expected the judges to make their decision in the next three days.
He said: "Our lawyers have told us we are likely to hear something in the next two or three days, which is fantastic as it is much quicker than we expected.
"Michael is pretty upset at the moment and obviously found the hearing very difficult.
"But he was given the chance to speak to the judges and he told them 'I am 100% innocent, I would not do this crime, I wouldn't hurt anyone'.
