"Sri Lanka stands on the brink," Brown said in a statement. "We have called repeatedly for the violence to cease.
"The humanitarian agencies must be granted access to civilians caught in the crossfire of a dreadful conflict. We are backing UN efforts to secure an orderly end to the conflict.
The LTTE must lay down its arms and allow civilians to leave.
"Sri Lanka must understand that there will be consequences for its actions.
A spokesman for Brown's Downing Street office told AFP the British leader would speak to Sri Lankan President Mahinda Rajapakse by phone later this weekend, and added that Brown had already called Rajapakse "several times to make his concerns clear."
Government forces in Sri Lanka captured the last strip of beach held by the Tamil Tigers on Saturday, leaving the