"Continuing in this government will be an injustice to the Sri Lankan Tamils," the former Tamil Nadu chief minister told the media.
Karunanidhi said the UPA government had not only refused to consider the DMK's views on the US-sponsored resolution against Sri Lanka at the UNHRC but had quietly watered down the resolution.
He also ruled out extending outside legislative support to the UPA.
Asked if the DMK would change its stand if the government brought a resolution in parliament denouncing Sri Lanka, he said there was time till Thursday for that.
If a resolution was introduced in parliament accusing Sri Lanka of committing "genocide" of Tamils, "we are ready to change our view", the DMK leader said.