The 35-year-old has enjoyed a glittering career at Stamford Bridge, collecting 11 major honours including three Premier League titles and the UEFA Champions League, but announced his intention to depart on Monday.
He also became the club's record goalscorer with a brace in a 2-1 victory over Aston Villa in May 2013 and will leave with 211 goals to his name from 648 appearances.
However, he was limited to 20 Premier League starts under Jose Mourinho this season, and found his place even harder to nail down after Nemanja Matic's January return to Stamford Bridge.
The former West Ham midfielder is a hugely popular figure at the London club, and stated he will offer a more thorough explanation for his departure to the club's supporters in due course.
But for now Lampard insists his focus is purely on England, for who he is preparing to feature at the upcoming FIFA World Cup in Brazil.
"It was very difficult (to leave Chelsea)," Lampard said ahead of England's friendly with Ecuador on Wednesday.
"After 13 years, the club was part of my life. It is difficult but you have to move on, the club will move on. I'll probably address it more to the Chelsea fans at a better time.
"I am here to concentrate on England. It was almost a relief to say it so I can move forward.
"I am really happy to be among an exciting England squad, full of skill, speed and exuberance."
Lampard's next move is as yet unknown, though he has been heavily linked with a switch to New York City, who will enter Major League Soccer next season and have already confirmed the signing of Spain striker David Villa.
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