The blast heightened the sense of alert in Brussels just as EU politicians, civil servants and diplomats returned to work after the Christmas break, which was marred by four devices in an apparent campaign against the EU orchestrated from Italy.
No one was hurt when the latest letter sent to German Hans-Gert Poettering, head of the European People's Party, the biggest group in the EU assembly in Brussels - was opened by a female, a spokesman said.
"The letter was opened... It exploded, (caught) fire and it was destroyed in the fire," said Robert Fitzhenry, spokesman for the party, adding that "the people that opened the letter described it as anonymous".
"Before it exploded, they did not see anything to identify where it came from," he said.
A statement added that the letter was from "an unknown source," adding that: "The security services of the European Parliament were informed immediately and an investigation launched."
EU head also targeted
European Commission head Romano Prodi, European Central Bank (ECB) chief Jean-Claude Trichet, as well as the heads of Europol and the crime-fighting organisation Eurojust in The Hague were all targeted by booby-trapped mail between Christmas and New Year.
The four devices were identical and according to the Italian press, contained a tract entitled Operation Santa Claus including anarchist symbols and signed by the Informal Anarchist Federation (FAI).
EU Chairman Romano Prodi (R)
also received booby-trapped mail
Nobody was hurt in those attacks, but prosecutors in the northern Italian city of Bologna, from where all the previous devices were sent, ordered postal authorities to block all mail sent to EU institutions from the city and surrounding region.
The latest security scares occurred despite tightened protection measures after the previous incidents. Officials are reluctant to discuss the details of measures taken to beef up security around EU institutions.
Shortly after the booby-trapped explosion occurred, a parliament spokesman confirmed that a second suspicious letter had been sent to another Euro-lawmaker.
The brown envelope, sent to Jose Ignacio Salafranca Sanchez-Neyra of Spain, was postmarked in Bologna on 22 December.
Aides immediately decided to call in Belgian police to investigate it, said the spokesman.
A spokesman for commission chief Prodi meanwhile reiterated that the EU executive was confident in the security measures already put in place.
"I'm sure that everybody understands that the Christmas and the turn of the year were not quite the peace and quiet we had hoped for," said Reijo Kemppinen.
"We are confident that everything that needs to be done has been done properly," he said.