Dubbed the Labour leader's 'cyber spinner', Smith was previously editor of the LabourList website and was an early backer of Miliband’s bid for the Labour leadership.
Smith now works in Miliband's private office in the House of Commons. He currently plays a key role managing online comms and blogger relations for the Labour leader, but his role also involves wider media relations and general comms.
Labour sources said Smith had informed Miliband of his plans and that he was expected to move on by the end of April. No decision has been made on how he will be replaced.
It is understood that Smith is still considering his next move, but could head to the US ahead of the elections there next year.
According to Labour sources, Smith plans to focus on writing and other projects in the short term. He has also begun working on comms for Future First, a social business building networks of former pupils prepared to return the classroom to give careers advice in state schools.
Smith confirmed he was planning to move on in the near future but declined to comment further.