At nearly 2am last Friday the young woman lying on a bench outside the bar in London's West End is so drunk she has passed out. She is in no state to notice, much less care, that her short skirt has ridden up – and she does not appear to be wearing knickers – or indeed that her Gucci handbag and credit cards are there for the taking.

Not a pretty or dignified sight, but more dangerous and disturbing is that the girl is easy prey for a passing attacker. Paramedic Brian Hayes shakes his head: "Look at the state of her. These young women just don't realise what risks they're taking when they go out and get smashed. They're so vulnerable."

Christmas is the busiest time of the year for the crew of what the medics like to call the London Ambulance Service's "booze bus". Tonight is no exception. The 30-year-old woman helped by Hayes turns out to work for an international bank. He checks her pulse, asks her to open her eyes and tries to get her into the ambulance. But she is unable even to stand.

Back in the ambulance, his colleague John Morgan has taken four more calls in as many minutes to scoop up Christmas revellers who have overdone the seasonal spirit. Antonia Gissing, the third member of the crew, is dealing with a trainee lawyer wearing silver cuff links who has vomited over himself and the booze bus. As she wipes his face and nose, he mutters insults. Almost paralysed with drink and slumped in the chair, he slowly, deliberately, curls his hand and waves his middle finger abusively in her face. "Fuck you, I pay your wages," he slurs before vomiting again.

"I've been spat on, punched, kicked, bitten, slapped… you name it," says Gissing. "It was frightening at first, but it happens at least once every shift and I'm used to it now. You can usually read the situation and guess when it's going to happen, but not always. The other night I gave a tissue to a girl who was crying and she bit my hand."

Morgan shows the gap where he lost three teeth after a drunk butted him a few weeks ago. Then the mobile rings again and we are off, lights flashing, sirens blaring; another street, another drunk.

The cost of treating each drunk is estimated at around £220 a time; the total cost to the NHS of treating alcohol-related injury and illness is thought to be about £3bn a year.

It was Hayes, a former soldier who served in Bosnia, who came up with the idea of the booze bus to take the pressure off the 999 teams. "I told the service to give me a couple of people and a vehicle, and I would go and pick up drunks from the streets. If you get a call between 11pm and 4am reporting someone on the street who is under 30 and isn't feeling well, you can more or less guarantee it's alcohol-related," he says.

Hayes says that, since the service started in 2005, the number of drunks has increased every year. This year the booze bus will scoop up around 70,000 people, 10,000 more than last year. The first calls start coming in around 9pm. "After midnight it just goes ballistic," he explains. "From 1am, accident and emergency will be choked up with people intoxicated with alcohol. There is no typical background. No class divide. It takes all sorts, from the 12-year-old drunk in the supermarket car park to the 55-year-old CEO of a West End company. This time of year you also have people who don't normally drink, going out and getting totally smashed."

A night shift with the booze bus is not for the faint-hearted or weak-stomached. Hayes, Morgan and Gissing have developed a coping mechanism that seems to consist of an almost unbroken stream of banter and black humour.

In a crowded West End nightclub few of the lurching dancers move to let the ambulance crew through. In a corner a young girl is slumped and unconscious. As the 25-year-old office administrator is led to the ambulance, a man she says is her boss becomes aggressive and jumps in. Gissing politely but firmly asks him to leave and there is a standoff as he first refuses, then goes. The woman insists she has not drunk very much. Finally she admits to three glasses of champagne, two shots of vodka and "maybe" two glasses of wine. "I think my drink was spiked," she tells Gissing, who has already heard this three times this evening. "No, love, you just drank too much," she replies as the young woman is sick into a plastic bag.

Patients unable to show they are safe to make their way home are taken to St Thomas' hospital near Waterloo. To ease pressure on casualty wards, the service is opening a temporary centre, where drinkers can be taken to sober up in the fortnight's run-up to Christmas.

In an evening, the booze bus has picked up several young professional women, bankers, businessmen, an ex-marine and a homeless person. A 38-year-old, well-dressed woman with a Cartier watch and Harrods leather coat is ordering the crew to get her a taxi. On a recent shift the bus picked up an off-duty policewoman who had been on an alcohol awareness course. It is, says Morgan, a quiet night.

The occasional drunk will apologise, and thank the crew, but most can barely speak. On the whole they are a lairy bunch and it's a messy, thankless job.

It is the lone women who worry the crew most. "For some reason the guys seem to stick together, but we often find a woman completely out of it and on her own. It's such a risk," says Gissing.

Click here for the London Ambulance Service's guide to a safe night out