Del. David L. Englin (D-Alexandria) told a meeting of Alexandria Democrats late Monday that he will not seek reelection in 2013 because he engaged in an extramarital affair.

Englin, 37, who is in his fourth term, told a meeting of the Alexandria Democratic Committee that he would leave politics and acknowledged that he cheated on his wife. Asked after the meeting if he would resign, he replied, “That’s a decision for another day.”

“Serving the people of the 45th District has been one of the great honors of my life, and I hope that you’re proud of the work that we’ve done together. But political life has affected me in ways that I never expected,” Englin told about 100 members of the Alexandria Democratic Committee’s monthly meeting at the Minnie Howard School in Alexandria.

“In my public role, I have worked hard to serve my constituents and to be a strong effective, progressive voice in Richmond,” Englin said. “But in my personal life, I let down my family, I was unfaithful in my marriage,’’ and he and his wife recently separated.

He said he would leave politics to focus on rebuilding his relationship with his wife and son.

Englin, an Air Force veteran who is a graduate of the U.S. Air Force Academy, joined the Virginia General Assembly in 2006. His profile rose steadily in recent years, as he became an outspoken advocate for liberal Democrats on behalf of gay rights, the environment and other progressive causes.

Englin was said to be among the possible candidates to succeed House minority leader House Minority Leader Ward L. Armstrong after Armstrong lost his reelection campaign. Instead, Englin backed Del. David Toscano to lead Democrats in the GOP-dominated House.

“I am deeply sorry for hurting the people I love the most and for disappointing so many others who have put so much faith in me over the years,” he told the meeting.

The 45th District also includes parts of Fairfax and Arlington counties.