Rice, who has Irish grandparents, played for the Republic at underage level and was capped three times at senior level in friendly matches in 2018.
But he has now decided to switch his international allegiance to England, the country of his birth, and plans to submit a formal written request to FIFA.
Rice said in a statement: “This afternoon I telephoned both Mick McCarthy and Gareth Southgate to inform them of my decision to submit a written request to FIFA for the transfer of my international registration from the Republic of Ireland to England.
“This has been an extremely difficult decision and, in all honesty, not one I ever expected to be making at this stage of my career. So much has happened, so quickly, in the past couple of years, from progressing through the West Ham United Academy to making my Premier League debut at the age of 18 in May 2017 and then being named in a full international squad just two days later.
“Like so many people around the world, I consider myself to be of mixed nationality. I am a proud Englishman, having been born and raised in London. However, I am just as proud of my family’s Irish heritage and my affinity and connection with the country.
“I have equal respect and love for both England and Ireland and therefore the national team I choose to represent is not a clear-cut, simple selection. Particularly not for a young lad who never dreamed of being in this position.
“Ultimately, it is a personal decision that I have made with my heart and my head, based on what I believe is best for my future.
“I fully accept that some Irish supporters will be disappointed by my decision, and that everyone has different opinions in regard to the rules around international representation. However, I hope that people can understand that I have made this decision with honesty, integrity and the full support of my family.
“I would like to thank Mick McCarthy, Martin O’Neill, all of the coaching staff and everyone associated with the FA of Ireland (with special thanks to Mark O’Toole), not only for their support and understanding throughout this period, but for the part they have played in my development as a young player.
“My pride at wearing the Irish shirt was always 100 per cent genuine. It was a great honour for myself and my family, and something I will always cherish.
“I would also like to make it clear that, in requesting to transfer my national team representation, I am not taking for granted a call-up to the England squad or making any assumptions about my future international career. However, out of respect for the Republic of Ireland, I felt it was right to announce this decision now and put an end to the speculation.
“In football, and in life, I have always tried to be completely honest and true to myself and my family at all times, and that is what I will continue to do.”
Republic of Ireland manager Mick McCarthy said: “Declan rang me today and said he has decided to give it a go with England. Good luck to him.”
Sky Sports News understand securing Rice’s pledge to play for England was one of the final tasks carried out by the FA’s former Technical Director, Dan Ashworth before he took up his new role at Brighton. Skysports