HONG KONG — Malaysian authorities have denied entry to a leading opposition journalist who is the sister-in-law of Gordon Brown, the former British prime minister.

The journalist, Clare Rewcastle Brown, who was sent back to Singapore, is the founder of the Sarawak Report and Radio Free Sarawak, two news outlets that have taken on the Malaysian government on issues like deforestation and corruption in the state of Sarawak, on the island of Borneo. A native of Sarawak, she has been in increasingly contentious battles with local power brokers and officials in the state since setting up the two news outlets in 2010.

In an interview on Thursday, Ms. Rewcastle Brown said she arrived in Malaysia on Wednesday at Kuching International Airport on an AirAsia flight from Singapore but was denied entry by immigration officials, who detained her and put her on the next flight back to Singapore. Ms. Rewcastle Brown, a British citizen who operates her news sites from London, said she had last been let into Malaysia in 2011.

Malaysia recently held democratic elections in which its prime minister, Najib Razak, was re-elected. Critics said the government used its strong hand over the country’s news media to help ensure that Mr. Najib remained in power. During the campaign, the Sarawak Report was often inaccessible because of what it said were cyberattacks.