Rajapakse and Blair were expected to discuss Sri Lanka's collapsing ceasefire with Tamil Tiger rebels, officials here said.
The trip comes amid international efforts to salvage the truce, officials added.
"The president left for London this morning," a senior government source said.
There was no formal statement from the government about Rajapakse's surprise departure at a time when government forces are battling Tamil Tiger rebels in the island's northeast.
Some 1,500 people have died since December, when Sri Lanka's three-decade-old ethnic conflict escalated again as a truce struck in February 2002 started unravelling.
The visit to London -- where Tamil Tiger chief negotiator Anton Balasingham is based -- is Rajapakse's first to Britain since he came to power in November.
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Britain, Sri Lanka's former colonial power, banned the rebel Liberation Tigers of Tamil Eelam (LTTE) in February 2001.
But Balasingham, now a British national, remains in London and holds meetings with fo