Sampanthan was seen holding up the national flag with opposition leader Ranil Wickremesinghe during the joint May Day rally.
Speaking at an event in Jaffna yesterday Senathirajah said that he received telephone calls from Tamils questioning the actions of Sampanthan, the Uthayan newspaper reported today.
Senathirajah said that he was not on stage at the time the flag was lifted up and that it was not part of the pre-arranged program schedule.
The TNA MP claimed that Tamils still want a separate state and will never accept the Sri Lankan flag.
He said he was saddened by what took place at the May Day rally and apologized on behalf of his party leader.
Earlier the Uthayan had quoted Mavai Senathirajah as telling the gathering at a May Day rally in Killinochchi that the army should withdraw from the area.