Using taxpayers' money to fund trade union activity cannot be justified "morally or economically", David Cameron has said.
The Prime Minister has reiterated his support for moves to stop the practice of government departments and public bodies paying salaries of staff serving as union officials.
The comments came in a letter to Tory MP Aidan Burley, who has launched a campaign to highlight the controversial issue.
The Trade Union Reform Campaign is carrying out research into the number of trade union activists subsidised by the taxpayer.
"I strongly believe the current level of public subsidy to the trade unions cannot be sustained, either morally or economically," Mr Cameron wrote.
"At a time when across the private and public sectors, people are having to take very difficult decisions in order to save money, it is difficult to justify some people in the public sector being paid not to do the job they are employed for, but instead to undertake full trade union activities - much of which should be funded by the unions themselves.
"We need to question why the public is paying for so much, and whether this is sustainable going forward."
Mr Cameron was replying to a letter from Mr Burley, who has clashed repeatedly with Labour MPs on the issue.
Earlier this week the Prime Minister promised action after it was claimed £113m of public money was being used to pay the salaries of union officials.
"I think the idea of full-time trade unionists working in the public sector on trade union business, rather than serving the public - I don't think that is right and we are going to put that to an end," he told the House of Commons.
However, unions have accused Mr Cameron of "kicking back" at officials who are doing a "valuable job". PA