Police have threatened to blacklist a 19-year-old British girl and deny her entry to Thailand indefinitely if she fails to counter investigators' findings that her rape claim on Koh Tao is groundless.

Having contacted London police, via the British Embassy, to question the girl, Thai officers expected it would take about a month to receive her account and evidence to back up her complaint, deputy Tourist Police chief Surachate Hakparn said on Sunday.

"We are waiting for her testimony," he said, insisting the investigators will treat the case in a fair manner to avoid making a one-sided conclusion. However, Pol Maj Gen Surachate, known as Big Joke, said evidence gathered so far did not support what the girl had told the British media about the incident on the island on June 25.

The Sun newspaper reported on August 23 that the woman claimed she was drugged, stripped, robbed and subjected to a nightmarish sex ordeal on Koh Tao. She then went to Phangan before returning to Koh Tao to file a complaint with police, whom she accused of refusing to investigate her rape allegation while noting only details of the robbery.

Police will consider her statement again but if "her words are not true, we need to press a charge against her on a false complaint, blacklist her and prohibit her from entering Thailand indefinitely," Pol Maj Gen Surachate said.

Police need to treat the issue carefully because some foreign tourists fabricated stories in order to claim insurance, he said.

So far there have been at least four cases in connection with false complaints -- two on Koh Tao, one on Koh Samui and the other on Krabi mainland, according to Pol Maj Gen Surachate. Police are taking legal action against the complainants, including informing the Immigration Bureau to put their names on the blacklist.

The officers are aware that Thailand is a popular tourist destination and revenue brought by foreigners is crucial to the country's economy, but it is also important to tighten screening. Up to 35 million travellers come to Thailand each year, making it one of the world's top destinations. "We must separate quality tourists from the bad ones," he said.