Democratic Left (DIMAR) was down to 12 MPs in the 300-seat House on Thursday after Thessaloniki MP Katerina Markou tendered her resignation in a letter accusing the party of being "old-fashioned, insular and plodding."
In her letter, Markou, who represents Thessaloniki's Second Constituency, also accused DIMAR leader Fotis Kouvelis of leading the party astray from "its original values and policies as well as from its ideological identity."
She wrote that she plans to maintain a "presence in developments," suggesting that she may be headed towards To Potami (The River), a new movement that emerged before May's local and European Parliament elections, and whose first congress last month was attended by four DIMAR MPs, including Markou.
Markou's resignation further comes after she refused to back DIMAR's position on a proposal for a referendum over the part-privatization of Public Power Corporation.
Sources suggested that Kouvelis may also be considering ousting MP Grigoris Psarianos for failing to back the motion, a move that would mean DIMAR shedding another seat to bring it to 11.
DIMAR was elected to Parliament in 2011 with 17 seats, forming a coalition government with New Democracy and PASOK. It left the coalition in June 2013 following a rift over the closure of state broadcaster ERT.
The party suffered a defeat in May's polls after seeing its percentage dwindle to 1.2 percent from the 6.1 percent it received in the general elections of June 2012.