Everton called a press conference for 7pm where the blockbuster transfer was to be confirmed but paperwork has delayed the announcement.
Belgium international Lukaku, 21, scored 15 Premier League goals on loan at the Toffees last season but will rejoin Roberto Martinez's side on a five-year deal, according to reports in his homeland and in England.
Lukaku, who joined Chelsea from Anderlecht in 2011, is expected to cost in the region of £25 million, nearly double Everton's previous transfer record, which was £15m for another Belgian, Marouane Fellaini.
The player had set tongues wagging with a cryptic Tweet showing him on an airplane, with the simple line "Time to write a new chapter".
The news was initially broken by Belgian football journalist Kristof Terreur.
Lukaku is rated as one of the best young strikers in Europe, having scored 17 games for West Brom the season before last.
He also scored for Belgium in the 2-1 extra-time win over USA in the World Cup second round, but has a frosty relationship with Chelsea boss Jose Mourinho, who has rebuked the youngster for speaking out about his future at the club.
Lukaku is undoubtedly one of the best young strikers, with a physique and presence that belies his tender years. He has only just turned 21, is intelligent, strong, quick and can finish - but Mourinho doesn't seem to like him, probably for personal reasons.
What is most surprising is not that the player is leaving Chelsea, but that a. he is not moving to one of Europe's bigger clubs, and b. the size of the fee, which is remarkable for an Everton which previously struggled to balance the books. It is something of a coup for the Toffees - who will fancy their chances of making the top four.
Romelu Lukaku celebrates scoring for Everton against Arsenal (Reuters)
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