Obama had planned to spend Monday night in Dublin and travel to London on Tuesday morning. But White House officials say the ash cloud changed course, forcing Obama to leave the Irish capital early.
Air Force One touched down in London late Monday. Aides say the president's schedule for the rest of the trip has not been affected so far.
The president and wife, Michelle, are in London for an official state visit at the invitation of the queen. Obama will also hold meetings with British Prime Minister David Cameron to discuss a range of international issues, including Libya and Afghanistan.