Fulham and Crystal Palace are investigating after Aleksandar Mitrovic and Luka Milivojevic appeared to break coronavirus rules.
The pair were filmed by Mitrovic's partner Kristina Janjic at a New Year's Eve party in London, with the footage posted on Instagram.
Fulham striker Mitrovic and Palace captain Milivojevic, who are both Serbia internationals, are seen celebrating indoors with at least seven other people, with the words 'Happy New Year 2021' accompanying the video.
Government rules state that people living in Tier 4 areas such as London 'must not meet socially indoors with family or friends unless they are part of your household or support bubble.