Diego Costa joined Chelsea from Atletico in 2014 £32 million and have been fairly successful for the Blues so far.
In his maiden season he had won the English Premier League title and also was the highest goal scorer for the club.
Recent rumours suggest, that Atletico Madrid manager Diego Simeone wants the striker back at the club. Costa improved and grabbed eye-balls of the football world under the same Argentine manager.
But as understood, the Blues are not particularly happy with how things are turning up and they would not tolerate any further enquiry about their star.Chelsea FC have threatened Spanish club Atletico Madrid that they will lodge a complain in FIFA if Atletico continue their pursuit for Diego Costa.
The Sun quoted a Chelsea source: "We're writing to them again, a second time, but if this continues there are further options open to us, which includes reporting them to Fifa. We sincerely hope it does not go that far."
Chelsea manager Antonio Conte too is not happy and will not let Costa leave the club under any circumstance.
Conte was quoted saying: "It is normal to ask for respect.
"I don't know this situation but I know myself, my attitude and behaviour, that I don't like to speak about the players of other clubs.
"I'd like that others don't speak of the players of my team. I know this period is strange. We respect all - but we want respect."
The Italian manager further added: "Diego has not put in a transfer request and is our player.
"He has shown that he is the strongest striker in the world. I'm sure that also when he gets into this idea of football he can score a lot of goals for us."
When asked if Costa was certain to stay at Chelsea next season, Conte said: "I have learned 100 per cent can never exist."