Cllr Neil Coyle - elected as Labour MP for Bermondsey & Old Southwark last May - has resigned his seat as a Southwark councillor for Newington ward.
Neil Coyle has represented Newington since he joined the council in May 2010. Labour took all three Newington seats at the 2014 local elections, with councillors Maisie Anderson and Eleanor Kerslake joining Cllr Coyle.
His resignation will trigger a by-election to be held on Thursday 5 May, the same day as the poll for Mayor of London and the London Assembly.
Official records show that Cllr Coyle has failed to attend 69 per cent of Southwark Council assembly (full council) and Borough, Bankside & Walworth Community Council meetings since he was elected to Parliament last May.
However, he has not claimed his allowance as a councillor since the parliamentary election.
During the 2015 general election campaign, Cllr Coyle told the SE1 website that he would quit the council if he felt that he "wasn't an effective representative" for Newington ward alongside his parliamentary duties.
In his resignation letter to council leader Peter John, Cllr Coyle said that "the balance of parliamentary and council work leave me in a predicament".
He continued: "With a baby due in less than two weeks and a new place on the Work and Pensions select committee, pressure will rise further rather than fall this year."
Cllr Coyle admitted that his ward colleagues councillors Anderson and Kerslake have been left with a "disproportionate burden which a new, third councillor would be able to alleviate".
"They need and deserve an equal partner in managing the ward's issues which I have been proud to have focused on for six years."
He added: "I will genuinely miss being a councillor and have enjoyed the experience immensely.
"To have been able to have served Newington ward for almost six years has been a real privilege and I am incredibly grateful to ward residents for their support and kindness in that time.
"I will continue serving them in Westminster to the best of my abilities and utilising the experience of local government that they have provided me."
Newington ward is mostly in SE17 and includes a small section of SE1 around the Elephant & Castle southern junction, including the Draper Estate and Strata SE1 tower.
During his tenure on the council, Neil Coyle has served in a number of roles include as a planning committee member, deputy cabinet member and deputy mayor.
In the south of the borough, Cllr Helen Hayes – elected last year as Labour MP for Dulwich and West Norwood – has also resigned her Southwark Council seat in College ward.