Jan Mokrzycki sits in his office in west London, reaches for a cigarette and reflects on 17 hectic months.

Ever since his native Poland joined the European Union on May 1 last year, Mokrzycki, president of the Federation of Poles in Great Britain, has been deluged with problems.

Other people's problems.

Poles come to him complaining about their treatment at the hands of unscrupulous employers. They call him to denounce the avarice of London recruitment agencies. They ask him to help in their legal battles with the British government.

"We've been very much busier since last year," Mokrzycki said. "We're dealing with a mixture of Polish bureaucracy and British bureaucracy and these difficulties pile up until they become a mountain."

Tens of thousands of East Europeans have used their countries' accession to the E.U. as an opportunity to leave home and head West in search of better jobs and wages.

And because most of the 15 old E.U. states have imposed restrictions on their labor markets, many have opted to come to Britain, Ireland and Sweden -- the only three of the 15 to have opened their markets fully.

The numbers are unclear, and hotly disputed.

Britain's Home Office, for example, says that in the first 14 months after accession, 232,000 people from the 10 new E.U. states signed up at its Worker Registration Scheme , as most are obliged to do once they have a job in Britain.

Of these, 131,000, or 57 percent, were Polish.

However, the figures do not tell the full story of movement across Europe's old Iron Curtain.

Many of those registered -- perhaps as many as 30 percent -- are believed to have been in Britain illegally before accession and simply registered to regularize their position.

Some signed up to take seasonal jobs and have since gone home, others have registered more than once, and still others, if they are self-employed, do not need to register.

Finally, inevitably, some immigrants avoid the WRS, and hence the British tax system, by working illegally in a thriving black market hungry for East European labor.

Buses roll into London each month carrying eager young workers from the former Communist bloc, and it sometimes feels like most pubs, cafes and hotels in the British capital are staffed by Poles, Lithuanians, Latvians and Slovenians.

"No one has the faintest ideas of the numbers," Mokrzycki said. "The Home Office says 131,000 Poles have joined the WRS. I would guess the true number is closer to 200,000, but, honestly, no one knows."

What is clear is that many more East Europeans have come to Britain and Ireland rather than to France, Germany and the other old E.U. states that have refused to open their markets.

Ireland, with its vibrant, flexible economy and small native workforce, has taken in 40,000 Poles, 18,000 Lithuanians and 9,000 Latvians in the past 12 months, according to the European Citizen Action Service, a civil rights watchdog.

By contrast, France, with 15 times more inhabitants than Ireland, issued just 1,612 work permits to Poles in the 11 months after May 1, 2004, despite French fears their country would be swamped by "Polish plumbers," a catchall stereotype of the low-wage East European migrant worker.

Magdalena Kierdelewicz, director of the Polish Information and Cultural Center in Dublin, said many Poles who came to make a quick buck on construction sites had opted to stay in Ireland and were moving their families over.

"A few months ago when you went on the plane from Poland to Ireland you saw only men, but now it's full of women and children," she said, estimating that there are now 80,000 Poles in a country of 4 million.

The benefits and drawbacks of immigration within the E.U. are, like the statistics, a matter of dispute.

The British government says eastern European immigrants contribute to the economy "while making very few demands on our welfare system or public services."

ECAS also says immigration has benefited Britain, Ireland and Sweden, providing them with efficient, cheap workers to do the jobs locals no longer want to do.

"The feedback we get from employers is that Poles have a strong work ethic, which is nice to hear," Mokrzycki said.

However, the pitfalls for workers are many.

"During our research we found some shocking examples of workers being exploited," said Julianna Traser, author of an ECAS study on migration within the E.U.. "In many cases, wages are low and workers have no written contracts."

While Britain and Ireland have opened their labor markets, they have refused -- unlike Sweden -- to open their benefits systems fully, meaning eastern Europeans have less of a safety net than their counterparts from the 15 "old" E.U. members.

"We had a case of a girl who was paid 1 pound 50 pence [$2.71] an hour in a potato chip factory," Mokrzycki recalled.

"She had a hemorrhage in her arm and fainted. She told her line manager about it and was sacked on the spot. No discussion, no back pay, no nothing."

Inevitably, though, many young Poles take their chances in the cut-and-thrust British economy rather than stay in their homeland, where unemployment stands at 18 percent.

"I came here just before accession, planning to stay for a few months, and I'm still here," said Monika, 25, as she served up steaming cups of coffee in a cafe in Hammersmith, the heart of London's Polish community.

Across the road, Grzegorz, a 27-year-old from Lublin, is scanning handwritten notes in a news agent's window in search of work and a place to live.

"I haven't got a job, but I don't think it'll take me long," he said. "I'm young, I'm fit, I'll consider anything. Who knows? This could end up being my home for the rest of my life."