The process, run through AAR, is in its early stages.
Agencies received requests for information on 19 July. It is not yet known when formal pitches will be held.
DLKW Lowe has worked with Legoland on a project basis since 2005, when it won a contested pitch against CHI & Partners and WCRS.
Also in 2005, the private-equity company Blackstone acquired Legoland for $375 million and packaged the business into Merlin.
Turnover at Merlin exceeded £1 billion in the 2012 financial year. The Legoland pitch comes as it looks to launch an initial public offering that could value the business at as much as £4 billion.
Merlin could not be reached for comment.