Pol Lt Gen Thaweesak Tuchinda, chairman of the Bangkok election commission, said the EC could not endorse Mr Sukhumbhand's victory as there are two complaints filed against him.
He said Bangkok's EC has accepted the complaints, one from a voter and another from a gubernatorial candidate, to proceed with the investigation and are now collecting evidence.
Gen Thaweesak said the two complaints alleged slanders during the election campaign, but he could not yet revealed detailed information until the Bangkok election commission meets this afternoon to discuss over the cases.
Mr Sukhumbhand contesting the Bangkok gubernatorial election under the Democrat banner defeated his major rival from the government’s Pheu Thai Party, Pongsapat Pongcharoen, by a substantial majority on March 3.
Meanwhile, Democrat MP Boonyod Sukthinthai on Thursday urged the EC to speed up the investigation.
He also asked the poll agency to apply the same standard two years ago when he filed a similar complaint with the commission accusing Red Shirt leader Natthawut Saikua of slandering the Democrat Party in 2011 election campaign and the EC ruled that the action was not unlawful as it was a personal opinion.
The EC secretary-general Puchong Nutrawong earlier announced that the EC meeting voted three to one to withhold Mr Sukhumbhand's endorsement to enable committee members to look into complaints charging Mr Sukhumbhand with breach of the Election Law.
If the investigation is not finished within 30 days, the EC will allow the governor-elect to assume the post while the probe continues, he said.