The incident occurred around 7.30 a.m. in the 50.5-km long tunnel linking Britain and France.
The evacuated passengers have been relocated at the French terminal, a Eurotunnel spokesman said.
Eurostar is a high-speed railway service owned and operated by Eurotunnel.
Eurostar’s customer care team said on its Twitter feed that “a section of the tunnel is closed, entailing single line working while extensive repairs are being carried through”.
The company has cancelled four trains between London and Paris and four more between London and Brussels.