The actors, who play Dennis Rickman and Sharon Watts respectively, will film their final scenes in November and leave the show in a sensational New Year storyline.
Speaking to DS today, a spokeswoman for the soap confirmed that Nigel has decided that he wants to leave for good, while Letitia is planning on taking an "extended career break".
Meanwhile, a source told The Mirror, "Writers are now planning Dennis' exit storyline. He has been a fantastic asset to the show and is one of the most popular characters. But he had made up his mind that the time was right for him to walk away."
Harman first joined the soap in 2003 and was voted Best Newcomer at the National TV Awards.