Some Premier League managers believe rival clubs flouted lockdown rules to hold training sessions following a huge gulf in fitness levels.
The top flight will resume on Wednesday night with two games before a full round of fixtures this weekend.
In preparation for the season's restart, clubs have been playing friendlies to help with match fitness and sharpness.
But that has exposed huge differences in the physical condition of certain clubs.
The Premier League was suspended in March shortly before the country went into lockdown.
Strict rules were put in place before they were gradually eased after several weeks.
Clubs were allowed to begin training once again last month, but they had to adhere to safety protocols.
Contact training was not originally permitted but the Daily Mail reports that some managers reckon their colleagues were holding secret sessions when they were not allowed to do so.
So much so that spies were sent out to rival training grounds to gauge whether certain teams were bending the rules to gain an edge.
The Premier League dispatched individuals themselves to ensure protocols were being stuck to.
But certain managers are still suspicious of the physical shape of some players.
Manchester City host Arsenal and Sheffield United play Aston Villa in the first two fixtures back.
Manchester United's trip to Tottenham is the headline clash of the weekend with their match-up taking place on Friday night.
Liverpool could also secure the title on Sunday should Pep Guardiola's side get beat at the Etihad.
If not they may have to wait a little longer to officially be named as champions.
