A synthetic chemical known as MDAI has already emerged as a successor to the drug mephedrone, which was banned in Britain this weekend.

Analysts at the Psychonaut Research Project, an EU-funded organisation based at King's College London, which monitors the internet for new trends in drug abuse, said it had identified the substance as the likeliest contender to replace the former "legal high". Co-principal investigator Paolo Deluca said: "Websites are already starting to promote MDAI and this could become the next popular product."

The drug replicates many of the effects of MDMA, or ecstasy, and was developed as an antidepressant by a team at Purdue University in the US during the 1990s. Experts believe its chemical blueprint could soon be mass-produced by the Chinese manufacturers who flooded the UK with mephedrone. Last year mephedrone became the fourth-most popular drug in Britain behind cannabis, cocaine, and ecstasy.

The many websites selling mephedrone closed down last week with the final deadline for placing online orders 3pm on Wednesday. At the same time, sites began advertising or offering MDAI, most describing it as a "research chemical". One of the most popular mephedrone websites states: "New products for April – MDAI." Another announced the arrival of MDAI by declaring it was "proud to introduce a new compound to our product range". The cost of the chemical is about twice that of mephedrone, with a gram – which users say is enough for five hits – costing £25.

Other online drug stores said they would soon sell substances that could circumvent the measures recently rushed through parliament to make mephedrone and its related compounds a Class B substance. Internet forums have surfaced in which MDAI users detail their experiences of the drug. Positive comments included a "snug" or "comfort-cloak" feeling; some described a "moderate serotonin burnout", or mild comedown. "Don't think it would be to everyone's tastes, there's no rushing/stimulation/euphoria like with other legals; it's more of a therapeutic feeling," one user said.

Deluca said: "Users will decide if it is worth the money. If it is deemed not as powerful as ecstasy or cocaine, then it might not become as popular. But because it is an antidepressant with research status, it will make it much more difficult to declare illegal."

The Psychonaut Research Project publishes reports to help states quickly identify new trends in drug abuse. The team identified mephedrone in 2008, long before it hit the headlines. Among the organisations that receive its updates is the US military so that officers can determine what drugs its personnel might be tempted to use when serving abroad.

Deluca said that researchers were examining Scandinavia, because drug takers there are more experimental and can provide an early indicator of new substances that will later become popular on a wider scale in Britain. The first reported death involving mephedrone was in Sweden in 2008.

Deluca said that, in addition to MDAI, it had also identified another synthetic substance – NRG-1 – that could become popular, but which the government's drug advisers are already looking at banning. Professor Les Iversen, chair of the Advisory Council on the Misuse of Drugs, recently said it intended to review NRG, which is widely prescribed in France as an appetite suppressant.