Young British women are being groomed online and offered cash incentives such as free travel to become "jihadi brides" according to an investigation in The Times.

The paper's three-month investigation, published without reporter bylines for security reasons, found that women as young as 17 are being offered cash to travel to Syria and marry fighters.

Journalists posed as two students, "Aisha", aged 17 and "Fatima", aged 19, to uncover how terrorists are using similar online techniques as paedophiles to attract British teens. The reporters found they were subjected to a barrage of online engagement, where friendship and a strong sense of community and identity was offered in return for betrayal and defection.

The Times reported: "Intelligence agencies and ministers are alarmed at the sophistication and impenetrability of jihadists' online activity and what they see as the inadvertent collusion of technology companies. It was with this in mind that The Times used subterfuge to reveal the ease with which teenagers are radicalised and the extent of Islamic State's financial muscle, stretching beyond the borders of Syria and Iraq and into Britain."

A Downing Street spokesman said social media companies had a responsibility to clamp down on grooming for terror as they did on paedophile grooming. "This underlines the real and serious risk of young people here in Britain being radicalised by terrorists in Syria. And it highlights why government, the security agencies, police and online companies all need to join forces against this common threat and do all we can to protect our children," the spokesman said. "We have already shown, through our co-operation to tackle online child exploitation, that we can do better at preventing access to harmful material online.

"Now we need to take action when it comes to getting extremist material down from the internet. We are making progress with the internet providers but there is clearly more to do."

Earlier this week, it was disclosed that police recently stopped a plane as it was taxiing down the runway at Heathrow to intercept a girl aged 15 who was attempting to join Islamic State. She had been planning to fly to Istanbul and then Syria. She was found just in time after her parents, who were unaware she had been radicalised, reported her missing. Another 15-year-old girl who had been on the same plane was not discovered.

In June, twin sisters Zahra and Salma Halane, both 16, of Manchester, left home together and reached Syria, where they married terrorists. They had 28 GCSEs in total and had been planning to become doctors. Their parents tried unsuccessfully to persuade them to return.

According to academics at King's, London, there has been a big increase in numbers of women leaving Britain for Syria.

Sir Bernard Hogan-Howe, the Metropolitan police commissioner, also said this week there had been increase in "people between the age of 16 and 18, and we've even had 15-year-olds...we've seen more young girls or women going."

The anti-extremism organisation Quilliam is tracking around 40 British jihadists who have left to join Islamic State, many of them girls aged 14 and 25.