Angry “non-domiciled” workers are threatening to leave Britain in protest at the Chancellor’s plans to clamp down on tax avoidance by charging a £30,000 flat fee to stay outside the country’s tax system.

The proposal will jeopardise thousands of jobs and lead to a brain drain, industry leaders have cautioned. Concern is growing that Alistair Darling’s proposal to charge a flat fee to non-doms who have stayed in the UK for seven years will spark an exodus.

One City-based Greek investment banker said the proposal meant he would not stay in London after seven years. “I’m not in London for the weather,” he said.

A Swedish banker said: “The plan has created a great deal of uncertainty. People are saying ‘what will stop the authorities continuing on that trend and increasing the fee?’ and questioning whether it’s worth it. I have been here for most of my working life but we may decide it’s not worth staying.’’

So far, the spotlight has fallen on the super-rich, including high-profile figures such as Sir Philip Green, the retail entrepreneur, and private equity bosses. But tax experts have said that middle-ranking wealthy professionals, who do not see themselves as part of the elite, are more likely to feel the pain from last week’s PreBudget Report.

Alex Henderson, an accountant at PricewaterhouseCoopers said: “There will be a class of people who may not even think of themselves as wealthy but who will be made a lot worse off.”

The shipping services industry said that the plans could drive away the international magnates on whose business the sector depends.

Jeremy Penn, chief executive of the Baltic Exchange, said that charging an annual fee to non-doms would jeopardise 15,000 jobs and significantly reduce earnings of £800 million in the ship-broking industry alone.

He said: “The maritime services industry is very valuable to the UK economy and a considerable portion of that arises from the fact an international group, particularly Greeks, is based here in London.”

David Asprey, the head of policy at the British Chamber of Shipping, said: “They make such an enormous contribution to London being the world’s maritime capital that we would be concerned about it. We would say, frankly, they’re making a bigger contribution by being here than they would by paying tax on their other business interests. Experience tells us that if you have a penal tax system on people who choose to be here they will choose not to be here.”

Individuals with links to other countries are allowed to declare themselves non-domiciled, exempting income earned abroad from tax. But after April, nondoms who have stayed in the country for seven years will have to pay a fee, expected to be £30,000, or pay tax on overseas earnings.

While the fee will be considered small change for high-profile super-rich non-doms such as Lakshmi Mittal, the steel magnate, and Sir Philip, the Top Shop founder, accountants say it will hurt many lower-paid foreign workers, including doctors and teachers.

It is estimated that an individual would have to have more than £1.5 million in assets abroad before it was worth paying £30,000 to retain non-dom status.

Mike Warburton, head of tax at Grant Thornton, said: “It won’t bother the super-rich because it’s not a lot of money for them and it won’t bother the Polish plumber, because they’ll be taxed on their British salary regardless. But it will be very relevant for people that have assets that they save up offshore. For example, one of my clients is an Indian doctor who has been prudent and saved up around £1 million, which will now be taxed because it’s not enough to make it worthwhile paying £30,000.”

An executive for a leading private bank said that a major concern was the prospect that, by electing to pay tax on their overseas income rather than a £30,000 flat payment, individuals could forgo other valuable benefits. “

At the moment, non-doms are also exempt from paying inheritance tax on their overseas assets,” he said. “It has not been explained whether non-dom status, and the exemption from inheritance tax, will be lost if they decide to pay tax.”