The number of crimes linked to two controversial mobile phone apps for people seeking casual relationships has soared.

Tinder and Grindr, two sites which have been accused of encouraging a casual sex culture, are increasingly figuring in reports or rape, robbery and extortion.

Police on both sides of the Border have warned of the dangers that dating sites in general are used by sexual predators and blackmailers.

But experts said a sevenfold rise in alleged crimes linked to Tinder and Grindr in just two years was "shocking".

They said users were vulnerable to "sextortion" and warned the figures may be "just the tip of the iceberg" as many victims will be too scared or embarrassed to contact police.

Victims are thought to include gay or bi men who are not "out" and people with partners.

Figures were obtained by the news agency by the Press Association (PA) using freedom of information laws.

PA had found just 55 reports of crimes in England and Wales mentioned Grindr or Tinder in 2013.

This jumped to 204 in 2014 and 412 in the year to October 2015, according to the 30 police forces who gave figures.

There were 277 crime reports in which Tinder was mentioned in 2015 - up from 21 in 2013. And 135 alleged crimes in which Grindr was mentioned were recorded in 2015, up from 34 reported in 2013.

Tinder is used predominately by heterosexual daters while Grindr is a gay dating app.

Reports of violent and sexual crimes were the most common, with 253 allegations of violence against the person and 152 reports of sex offences, including grooming, rapes and the sexual exploitation of children.

Neither Tinder no Grindr would comment.

The figures come from police reports where the apps are mentioned in the crime report and does not automatically mean the app was used directly by the criminal.

Former Scottish Chief Constable Sir Stephen House in 2014 warned that dating apps - he did not specify which - were cropping up in sex crimes reports "on a weekly basis".

He add: "We are dealing with sexual assaults that are directly related to people who met in virtual space and may or may not have presented themselves truthfully to one another.

"They take the step of meeting in reality, where it quickly deteriorates into sexual assault: we have an offender and we have a victim."

In England, the National Police Chiefs' Council's lead on violence and public protection, Deputy Chief Constable of Merseyside Police Andy Cooke, said: "The rising popularity of online dating apps and websites has contributed to an increase in the number of recorded crimes.

"I would urge those who use online dating apps to be as security conscious as possible and not to share personal data with anyone until they are sure about those they are communicating with.

"Similarly, individuals should stop all communication with anyone who attempts to pressurise them into something they are not comfortable with."

Andy Phippen, professor of social responsibility in IT at Plymouth University, said there was a "growing problem, particularly around sextortion-type activities".

He said: "If we are going to base the formation of a relationship on a photo and a few lines of text, how do we know that person is who they say they are and they have the right intentions?

"It is something we tell kids about - don't trust who people say they are online - but the adult population go merrily about their way thinking it is a good way of hooking up.

"And there will be people who are using these apps to coerce and groom children, sadly."

Last August, Daniel Edwards and his partner Kristofer Wagner, from Gloucester, were jailed for blackmail after threatening to expose a married man they met on Grindr.

And former teacher Gary Pearce, from Sidcup in Kent, was jailed for five years last September for grooming a 14-year-old boy he met on Grindr.

In September 2014 the Garda in Ireland said officers in Dublin had arrested a man on suspicion of raping a woman he met through Tinder.