The 26-year-old Czech Republic driver's agent, Antonin Charouz, confirmed the nature of the substance on Friday.
Enge, who lies in second place in the Formula 3000 drivers' championship, maintains his innocence.
"Tomas could have breathed it in at a disco," said Charouz.
"Someone could have slipped him it, baked it into something he ate or drank."
Enge, who drove three races for the now defunct Prost team in 2001, has been summoned to appear before the next meeting of the World Motor Sport Council in Paris on 1 October.
Shocked
Charouz and team principal Christian Horner both insisted Enge will race this weekend.
"Tomas will take part in the F3000 race in Monza but his result will be unofficial," said Charouz.
Horner added: "Tomas is insistent that he hasn't knowingly taken any illegal substance or drug and is completely shocked and surprised to have been tested positive.
"Tomas was surprised and shocked by the report. He will ask for second tests and will be starting in practice Friday," Horner said.
Enge is one point behind Formula 3000 leader Sebastien Bourdais of France heading into the final race of the season at Monza.
If he snatches the drivers' crown on Saturday then the result will be provisional until the World Council hearing next month.
The Arden International driver can request a re-test of the original sample.