BREGA, Libya — Libya's foreign minister defected to Britain on Wednesday, dealing a blow to Col. Muammar el-Qaddafi's government even as his forces pushed rebels into a panicked retreat and seized valuable towns they ceded just days ago under allied airstrikes.

The defection of Moussa Koussa, the foreign minister, showed that at least one longtime confidant seemed to be calculating that Colonel Qaddafi could not last. The news of Mr. Koussa's defection sent shockwaves through Tripoli on Wednesday night after it was announced by the British government.

The government advance appeared to return control of eastern Libya's most important oil regions to Colonel Qaddafi's forces, giving the isolated government, at least for the day, the east's most valuable economic prize.