40 MPs from the Union of Greens and Farmers, National Alliance, and For Latvia From The Heart supported the bill. Voting against were 22 members of Unity and Regional Alliance, while another 29 MPs from Harmony and Unity abstained.
The Revenue Service's acting director general Dace Peleka earlier said that the merger would reduce the number of police chiefs but increase the number of investigators.
MP Andrejs Judins (Unity) said during the debate on the law that while Unity agrees with the proposed merger, the party objects to handing control over the Interior Security Department of the Finance Police to the director of the State Revenue.
Opposition MP Mārtiņš Bondars (Regional Alliance) said it'd be better if control over the interior security force were handed to another institution, like the Corruption Prevention and Combating Bureau.
Several MPs criticized the proposal in the amendments to make the Revenue Service head wear a uniform.
While Edgars Putra (Union of Greens and Farmers), Parliamentary Secretary at the Finance Ministry, said that turning down the amendments would amount to voting against curbing corruption, VAT fraud schemes and against sacking unscrupulous employees from the tax service.
At the moment, there are two investigative departments at the Revenue Service - Finance Police and Customs Police.
The amendments have stirred disagreement within Latvia's ruling coalition.
Finance Minister Dana Reizniece-Ozola told LTV on Wednesday that the amendments had "stuck in Saeima" because of Unity, while Unity criticized the legal quality of the amendments.