Introduction: Atrial fibrillation (AF) is the most common arrhythmia in adults and is associated with a higher risk of heart failure or death. Automatic detection of AF in ECG is still problematic, as was shown by the results of previous studies. Here, we introduce simple and efficient method for automatic AF detection based on symbolic dynamics and Shannon entropy. Methods: This method comprises of three parts. Firstly, QRS complex detection is provided by detector based on phasor transform, and sequence of RR intervals is computed. In the second part, the raw RR sequence is transformed into a sequence of specific symbols and subsequently into a word sequence. Finally, Shannon entropy of the word sequence is calculated. According to the value of Shannon entropy, it is decided, whether AF is present in the current cardiac beat. Results: We used four publicly available databases for testing of our algorithm. We achieved sensitivity Se=96.32% and specificity Sp=98.61% on MIT-BIH Atrial Fibrillation database, Se=91.30% and Sp=90.8% on MIT-BIH Arrhythmia database, Se=95.6% and Sp=80.27% for Long Term Atrial Fibrillation database and Se=93.04% and Sp=87.30% for CinC Challenge database 2020. Discussion: The achieved results of our one-feature method are comparable with other authors of more complicated and computationally expensive methods. Our ECG experts found that public databases contain errors in annotations (in sense of AF). It means that results are affected by errors in annotations. Many errors were found in Long-Term AF database, several also in MIT-BIH AF database and MIT-BIH Arrhythmia database. Testing algorithms on inaccurately annotated databases cannot bring reliable results and algorithms for real medical practice. The examples of such annotations are reported in this study.