Britain's Prime Minister David Cameron promised back in 2010 to bring net migration down to 100,000 people a year. Six years later, it's more than three times that number.

That's one reason the government's Home Office decided that non-Europeans on skilled worker visas — known as Tier 2 visas — are not welcome to stay unless they are making at least 35,000 British pounds (about $50,000 a year).

The message is aimed at slashing migration to Britain and goes into effect in April. But critics call the new rule discriminatory and say it will strip Britain of lower-paid artists, health care workers and tradespeople.

"The estimates put the GDP loss at 181 million [pounds, equivalent to $264 million] to 761 million [pounds, equivalent to $1.1 billion], so that's a massive blow in the first year alone, for starters," Joshua Harbord says.

Harbord, who rattles off facts and figures about why he thinks this move will hurt Britain, might sound like an expert on immigration. But he's not. Harbord performs at kids' birthday parties as a pirate. And when he heard his friend Shannon Harmon might be forced to leave, he got angry.

"It was scaring Shannon and everybody it was affecting, and it felt like a massively mean policy that was apparently convincing my friends that they were worthless and unwanted," Harbord says.

Since no one else was doing anything about it, Harbord started a petition, Stop 35K, to try to change the government's mind. Harmon, who is from Chicago, has a work visa and has been in the U.K. more than seven years. But she makes less than the amount needed to stay under the new rules.

"It feels pretty horrible and unfair; that's why we're trying to fight it," she says. "I don't think we should be valued on an arbitrary number that they've made up. I mean, not that many people make that much money."

Harmon works for a nonprofit and says charity workers, who aren't paid well, contribute more than those in the finance industry. The changed rules will also affect health care workers, public transport workers and the many artists, musicians and actors who are drawn to London as a cultural hub.

"It's just going to make places like London less dynamic, less cultural; it's just going to change the whole atmosphere," Harmon says. Come April, she will very likely have to leave her British life partner, her career and the U.K.

"I leave everything, my whole life," she says.

Harbord and Harmon spend their nights brainstorming, working on their website and checking on the electronic petition. Every few minutes, the signatures increase. They are approaching the 100,000 they need for Parliament to consider debating the issue.

At present, immigration is a hot-button issue in Britain. There is another petition calling on the government to completely close the U.K.'s borders. And last weekend, rival protests supporting and opposing immigration turned violent.

David Metcalfe, chairman of a committee that advises the government on immigration, recommended the plan that is scheduled to take effect in April.

"It seems to me absolutely right," he says. "They've been here five years. If they're going to settle, they should be making a proper contribution in terms of productivity, which will be reflected in their pay."

Asked about critics who say it's an arbitrary measure that values money over other contributions to British society, he says: "Pay, in my view, is the best measure of skill and contribution, but you are right, it's not a perfect measure."

Metcalfe says there will be temporary exceptions for people with skills such as nursing, because there is a shortage in the U.K.

Susan Cueva, however, has been trying to stop the new rules. She works with UNISON, a trade union that represents the public sector — people who work in education, health care and transport.

"It's a policy that is not really based on sound judgment," Cueva says. "I think from our point of view as a union, we always look at migrant workers as an asset and a resource in the country."

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