Britain branded Syrian President Bashar al-Assad's election victory an "insult", saying Thursday the poll was merely a way of "sustaining his dictatorship".

Foreign Secretary William Hague said Assad lacked legitimacy before the election and lacked it afterwards.

Assad, who succeeded his father in 2000, was re-elected with 88.7 percent of the vote. Tuesday's election was labelled a farce by rebels fighting to overthrow him, and its outcome was never in doubt.

"Assad lacked legitimacy before this election, and he lacks it afterwards. This election bore no relation to genuine democracy," Hague said in a statement.

"It was held in the midst of civil war with millions disenfranchised, denied access to basic humanitarian aid, and with all opposition to Assad brutally suppressed.

"Holding an election in such circumstances is just a way of sustaining his dictatorship, and is an insult to the Syrians who have been calling for freedom and real political change."

Hague said Assad had no plan for peace, stability or reconstruction in Syria.

"His only formula is the killing and starvation of his own people, the destruction of whole cities and the displacement of millions," Hague said.

"He is pushing Syria towards greater fragmentation and further misery."

He said Assad had a clear route to end the conflict that has been raging since March 2011: engaging with the United Nations-sponsored Geneva process which calls for a transitional governing body established with the mutual consent of Assad's regime and the opposition.

Hague said Britain was increasing its support to those seeking a "genuine political settlement" in Syria, including by "backing the moderate opposition and their vision for Syria of true democracy, pluralism and respect for human rights".