Political novice Yingluck Shinawatra says she has formed a coalition with four smaller parties, boosting her majority in parliament a day after being elected Thailand's first female prime minister.

Election officials said Ms. Yingluck's Puea Thai party captured 265 of the 500 seats in Sunday's voting. The addition of the smaller parties will bring the total to 299 seats, providing insurance against legal challenges that could eat away at the Puea Thai majority.

The outcome was a stunning setback for the Democrat party of Prime Minister Abhisit Vejjajiva, who announced Monday he will step down as party leader.

It also means a return to influence for Ms. Yingluck's older brother Thaksin Shinawatra, a former prime minister who was deposed by the military five years ago. Despite their hostility to Mr. Thaksin, military leaders said Monday they will respect the election result.

Mr. Thaksin has been living in self-imposed exile in the Middle East because of a conviction on corruption charges that he says was politically motivated. One of the first challenges facing Ms. Yingluck after the new parliament is seated will be whether to pardon her brother.

She must also decide whether to seek charges against military leaders over last year's political violence in which more than 90 people were killed, most of them “Red Shirt” supporters of Mr. Thaksin and his political allies.

Mr. Thaksin openly advised the Puea Thai campaign from exile and selected his 44-year-old sister to run it. However, the successful businesswoman proved to be a very capable campaigner and quickly won over many doubters.

Ms. Yingluck promised Monday to pursue a policy of reconciliation, seeking to end tensions between her brother's poor and largely rural supporters and the Democrats with their better educated and pro-monarchy followers.

The United States issued a statement late Sunday in Washington congratulating Thai voters for their “robust participation” in the election.