Football officials in Rio de Janeiro said the Portuguese, currently plying his trade for Brazilian top flight club Fluminense, failed a doping test conducted on March 30 following his side's 3-0 win over Boavista.
The 35-year-old, who played two seasons for Chelsea between 2008 and 2010, has argued that the traces of furosemide - a diuretic which can help hide performance enhancing substances - came from contaminated vitamins he had been given by a pharmacist.
"On the reports linked to midfielder Deco's antidoping test, Fluminense will only make a statement on Friday, May 3, when the B sample comes out," the Brazilian champions said in a brief statement on their website.
"My career goes back 18 years, 15 of them in Europe, without any problems," said Deco.
"I've always taken vitamins and never made use of anything illegal. I spoke to my lawyer and I'm going to wait for the B simple before saying anything. I'll go to as far as necessary to clarify the facts," he was quoted as saying by Gazeta Esportiva.
In the meantime, Deco has not travelled to Ecuador with his team-mates for the first leg of their Copa Libertadores knock-out stage tie against Emelec.
Deco made over 40 appearances for Chelsea before moving back to the country of his birth with Fluminense in 2010.
He began his career at Corinthians before making his name during a five-year spell at Porto, during which time he won both the Champions League and the UEFA Cup under the tutelage of Jose Mourinho.
A move to Barcelona followed and he added a second Champions League title to his impressive CV in the Catalan capital, along with two Liga titles.
At Stamford Bridge, the playmaker won the Premier League and two FA Cups before returning home with Flu on a free transfer.