Labour's national executive committee was contacted by thousands of people backing its decision to boycott security company G4S over its ties to Israel.
According to one source, as many as 3,600 messages were in support of the boycott.
It is understood that initially only around 100 pro-Israel supporters emailed the committee to oppose the move.
The NEC voted to boycott G4S by 12 votes to four last month because of the firm's work in Israel and the Palestinian territories.