Prague, Amsterdam, Riga, Krakow, they have all become popular destinations for the British stag party industry. But some of the estimated £500m spent each year is also finding its way to criminal gangs trafficking women for the sex trade.
The British stag party has changed in the past 10 years. The drink down the pub with dad or your mates is gone. It has been replaced with three or four nights in a foreign city, far from prying eyes and geared to meet the stag party's every desire.
And that is often much more than a visit to a strip club. For a large number of stags, visiting a prostitute has also become part of the ultimate lads' weekend.
Prague has 70 brothels and numerous strip clubs. It is not hard to find some of the half a million stag visitors, many British, that the Prague tourist board say visit every year. I found them at every sex establishment I went to.
There was a strip party bus, lap-dancing, and even a brothel that offers free sex provided you agree to be filmed. And many of the British stag groups had at least some, if not all, members buying sexual services.
A local charity worker was in no doubt that British stag parties coming to Prague had increased local demand for sexual services.
"If a British stag sleeps with women in a brothel or sleeps with her on the streets it could be a women brought because of the sex industry, to be trafficked there from any part of the world."
The Czech Republic has long been a transit point for the trafficking of women in and out, and I wanted to find out how easy it would be to buy trafficked women for the sex trade.
Through contacts in the criminal underworld, we managed to arrange an undercover meeting with a trafficker. Posing as a London brothel owner, I arranged to meet him at a motorway service station.
As I secretly filmed our meeting, it became clear how straightforward the deal would be.
"I have two girls here for you now. That I can give to you. They have papers, they can travel you know, no criminal record, anything," he told me.
In the world of international sex trafficking, the leasing of girls has become common and it is big business.
"One girl makes me 90,000 krona (3,500 euros, £3,119) a month - around 50,000 euros (£44,561) a year. You give me 3,500 euros per month and I will come and visit you every month in England and collect the money every month."
The meeting lasted 20 minutes, and I could have bought two women.
In the Latvian capital Riga, brothels are illegal, so the sex industry is more discreet with back-street brothels and street workers.
One British man told me about a "back alley basement place" his group had visited.
Like almost all the Brits I met, he did not want to tell me his name or be identified, admitting they were doing things they would not dream of doing at home.
But he said they had been shown a group of girls and he thought they were probably trafficked.
"They stood against the wall with a lack of soul, a dead look on their faces, which suggested that they weren't necessarily there out of their own free will.
"My initial feeling was they had just been brought out of the cupboard and they've just been released from the shackles and marched out to then have an hour with someone."
He told me half the group had seen enough at that point and left. The other half stayed.
Poverty and desperation
Far from the bright lights of the stag cities is Lunik 9, a decrepit crumbling estate in the Slovakian city of Kosice.
The centre wells of the tower blocks are six metres (20ft) deep with rubbish, the windows of the apartments have long since vanished and sewage runs down the walls and through the streets.
It really has to be seen and smelt to be believed, and it is here, amid this poverty and desperation, that sex traffickers look for and then exploit their victims.
Yana was only 17 when she left Slovakia with a man she thought was her boyfriend, to start a new life in Amsterdam.
When she arrived, she found she would not be leading the life that she dreamed of.
Unknown to her, her boyfriend had sold her to brothel owners.
"He led me to a space where there were a couple of men who were saying that I was beautiful, that I was sexy and they were basically saying that she is here for us, this is what we want her to do They tore apart my clothes and they raped me."
Yana provides a tragic and deeply emotional testimony of being raped daily and pushed to the brink of suicide.
It is a story that every stag should hear.
Yana finally escaped after a year trapped in Amsterdam.
Tibor, another sex trafficker from Slovakia but working in the Czech Republic, agreed to be interviewed about his operation.
It was not his openness that stunned me but his cold, detached attitude.
I asked if he viewed the trafficked girls as his employees.
"More like things that I own," he said.
He said he felt no guilt about making money from people who were trapped. "It's just a regular job where you go to work and you go home at the end of the day."
It is estimated that more than 3 million British people go on stag and hen parties each year, with more than 70% of them going overseas.
Amsterdam is the most popular British stag party venue. The local sex industry largely depends on foreign women from poorer countries.
There were nearly 500 women rescued last year according to Dutch police.
"Each year we have more cases," I was told by a policeman who runs an undercover team that infiltrates trafficking gangs.
The industry is worth "millions of euros", he said. "The recruiters are getting money, the transporters are getting money, the exploiters are getting that money."
"Only the girls will get nothing," he adds.
In an attempt to make the area easier to police, the local authorities are forcibly buying up nearly half of the 482 brothels in the red light district and closing them down.
But it is not difficult to find a British stag weekender, a bright young man who had admitted to using a prostitute.
I told him that many prostitutes in Amsterdam had been trafficked.
"I don't agree with that, I think it's awful," he said, but admitted it probably would not stop him using a prostitute again that night.
"I disagree with it, but at the end of the day it's just what happens, like. You can't do anything about it. You're just here to have fun, and do things you won't get away with back home."
Simon Boazman reports in Stag Weekends: The Dirty Secrets on BBC Three on Thursday 14 January at 2100GMT or afterwards via the BBC iPlayer .