Despite Conservative demands, the Treasury has refused to rule out changes in the tax regime for expatriates.

The Tories have accused the chancellor of planning a third term raid on non-resident nationals' incomes after his department did not deny the possibility.

Party co-chairman Liam Fox wrote to Gordon Brown following a to trip Hong Kong, Dubai and South Africa where he met with Tory supporters who expressed their fears.

Fox asked him to offer an assurance that ministers would not target Britons living abroad for extra revenue, arguing that the move would hit exports.

At present expatriates do not pay income tax on money earned overseas as long as they spend only 90 days or less in the UK.

However some non-residents are thought to fear the Treasury could reduce this limit to just 30 days or introduce an American-style system whereby the difference between domestic and foreign taxes is levied if the latter is lower.

Brown has said the "complex and far-reaching" issue will be reviewed after consultation.

But a Treasury statement insisted that "tax policy is made in Budgets not in response to Liam Fox's overseas visits".

Fox said the response was evidence of the government's need for more money to fund its spending plans.

"It is clear from the Treasury's lack of a denial that it is looking at new ways to milk British taxpayers whether they live in the United Kingdom or not," he said.

"This will only add to the impression that Gordon Brown is identifying tax rises should Labour win a third term. No taxpayer will be safe whether they live in Britain or not."

The Labour Party accused him of doing the bidding of wealthy Tory donors.