Dutch MEP Sophie in 't Veld had invited Sir Iain Lobban, the director of British signals intelligence hub GCHQ, to attend the event in Brussels on Thursday.
But Sir Jon Cunliffe, the British ambassador to the EU, replied in a letter seen by The Daily Telegraph that she had no right to do so under the EU treaties.
"National security is the sole responsibility of member states. The activities of intelligence services are equally the sole responsibility of each member state and fall outside the competences of the Union. For that reason, and with respect, the UK must decline your invitation," he said.
Referring to the Belgacom allegations "it is my government’s consistent policy not to comment on intelligence matters".
A spokesman for the British embassy to the EU added that it was not a matter of "being nice or not" to the EU parliament. "It's a matter of national security, not a European matter," she said.