Is there a penalty for deliberating losing or fielding a weakened side?

Question

In the English Premier League, if any team deliberately tries to lose a game to give the opposition more points that could possibly damage the league position of a third team, say by fielding a weakened side or in extreme case score own goals, is there any penalty they could face for such action?

Also, I heard few years back that relegation some candidate threatened to report Chelsea to FA because they fielded a weakened side with young players against their relegation rivals at the end of the season game.

Is there any particular criteria set by FA to determine just what constitutes a weakened side in such a case, since the strength of a team is very subjective? Secondly has there ever been any action taken for such an incident in past?

EDIT: (Could find an example for EPL but here's a hypothetical example for CL group stage where losing is beneficial)

In the final two matches Roma play Celtic and Barcelona play Shaktar. Barcelona had a poor run in previous five games due to injury to key players and silly red cards. Assuming that Barcelona can beat Shaktar in the final game, it makes sense for Roma to lose to Celtic deliberately to make them qualify instead of Barcelona, so that they wouldn't have to face a full strength Barcelona later in the group stages when all their key players have recovered.

Answer 1

Since 2011, teams have been allowed to field any combination of their 25 man "match day" squad without fear of reprisals from the Premier League. Previously to that, teams were fined (relatively) small amounts for infringements, for example Blackpool in 2010 or a suspended fine for Wolves in 2009. Teams can in theory still be punished if they field weakened teams with players from outside their 25 man match day squad.

If a team does anything like deliberately scoring own goals to benefit another team, they can expect to have a ton of bricks dropped on them by both the Premier League and the FA for bringing the game into disrepute, barring exceptional situations like that which occurred in Doncaster vs Bury in August 2015, when Doncaster allowed Bury to score an equaliser after accidentally scoring when returning the ball after an injury.

Answer 2

I think you're mixing two different issues together.

1. Deliberately losing the game is unsportsmanlike behavior, puts the intended third team under unjust competition conditions and will be punished by the regulatory body, very possibly severely (point deduction and even relegation). Scoring a deliberate own goal is one of such an obvious behavior.

2. Fielding an under-strength team, on the other hand, is allowed. However, in most cases it's not because the team want to deliberately lose the game. It's because of the tight schedule and thus the need to rest key players and rotate the squad. This is why a team has 25 players registered, not 18.

Of course, you might argue that if a team really wants to deliberately lose, it can still deliberately field all its substitute players even when the starting players are healthy, and play sluggishly without actually putting the ball into their own net. Such a behavior would be muddier to judge. However, I don't think there would be any case where a team really wants to lose a game on its own: it simply makes no sense for them. If that happens, it's almost surely because of bribery from another team, and in such a case it would certainly be a breach of rules.