With its crystal clear waters, white sandy beaches and iridescent sunsets, the Maldives is a sought-after destination for loved-up British honeymooners. But its Edenic reputation is under threat after a teenage rape victim was sentenced to 100 lashes for engaging in premarital sex. Now justice campaigners are planning to target the country's lucrative tourism industry unless it improves womens' rights.

The ultimatum has been issued by the world's largest online campaigning group, Avaaz. The network, which boasts 20 million members, has designed an advert contrasting the country's sunny beaches with an image of a crying young girl and a reference to the island republic's Islamic laws. The advert will be published in the travel press, online and via social networks if politicians do not agree to change legislation, according to the organisation.

The charges against the girl, from the tiny island of Feydhoo, were brought by police who said she confessed to engaging in sexual activity outside of marriage. The confession reportedly came when officials were investigating separate accusations that her stepfather had raped her and killed their baby. Maldives sources said the girl is thought to have been abused by local men for some time. Her mother is also alleged to have concealed the crime.

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The flogging sentence prompted worldwide indignation from human rights groups which condemned the punishment as "cruel, degrading and inhumane". The Virgin tycoon Sir Richard Branson said the sentence was causing "enormous damage" to the Maldives, after asking the country's President, Mohammed Waheed Hassan, to intervene.

The threat of Avaaz's advertising campaign – used successfully in the past against the Hilton Group – is designed to increase the pressure on the government to review legislation. Around 35 per cent of the Maldives' economy is directly linked to tourism. Mohamed Maleeh Jamal, the deputy tourism minister, said he was "extremely worried" about the impact of adverse publicity on the industry. Around 100,000 Britons travel to the Maldives each year, making the UK its third-largest market for tourism.

Alice Jay, campaign director at Avaaz, said: "Britain's number-one honeymoon destination is a place where torture of women and girls is the law. Holidaymakers have a right to know what sort of regimes they're supporting and to make their travel decisions accordingly."

The islands' legal system combines elements of Sharia law and English common law. More than 100 "fornification" cases were filed in 2011, with almost 90 per cent of those sentenced women.

Masood Imad, a spokesman for the President, told The Independent on Sunday the case had been "blown out of proportion." He said: "The government is already engaged with the judiciary and trying to get the sentence repealed. The Government thinks flogging should end and that we should try and come up with a more modern and appropriate way of dealing with things."

He added that it was not the Government's role to "mess around with the judiciary," but that the law itself must be changed. Ms Jay said that Avaaz wanted to see "a bill that upholds girls' and women's rights on the tables in parliament".