Is it permissible to complete a voluntary fast if the intention was made and suhoor was eaten, then it was discovered that the Fajr adhan had already passed?

If you eat the pre-dawn meal (suhoor) after the Fajr (dawn) prayer time has begun, the fast of that day becomes invalid, whether it was an obligatory fast or a supererogatory one. However, it is recommended to complete the fast of that day and then make up for it (by fasting another day).