Bill Morris, general secretary of the Transport and General Workers Union, criticised the plans saying they were damaging the morale of staff.
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The trade unions represent their members. They are right to express their concerns about it
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John Prescott
Tony Blair signalled during the general election campaign that he was prepared to hand over many of Britain's schools and hospitals to private contractors.
But Mr Morris said the policy "almost guarantees to make the National Health Service look like Railtrack on a bank holiday".
Writing in The Observer newspaper, he criticised the fact that the policy was becoming, in his view, an ideology.
"The government has said there should be no ideological bar to a role for the private sector in delivering public services.
"I for one reject the notion that efficient public services can only be provided in partnership with the private sector.
"It is a belief which is fast becoming an ideology - one which does immense damage to the morale of hard-working public sector employees."
Union dismay
His comments reflect the dismay among the public service unions at the extent of Labour's manifesto plans to use private companies to improve public services.
Deputy Prime Minister John Prescott said the government was ready to listen to unions' concerns but would not allow them to block the plans.
He told BBC One's Breakfast with Frost programme: "Public-private finance has been an important initiative. It is the best use of public and private sector financing within a number of areas.
"It may be in hospitals, in some cases, and it certainly will be in transport.
"In hospitals, the radical principle is treatment free at the point of need, based on your need, not your ability to pay.
"The radicalism that follows from that is putting in more resources. Where there are facilities in the private sector, for example in Bupa, there is no reason why we can't take these private beds and say it's an extreme emergency and use them and contract out for them.
"There is no reason why you can't mortgage buildings from the private sector to build a hospital ... but the staff are employed by the NHS and the principle remains the same.
Mr Prescott said the government would press ahead with the plans despite the opposition of unions.
"The trade unions represent their members. They are right to express their concerns about it.
"In some cases, they are perhaps against change sometimes and we must have a debate with them. That debate will continue.
"But that is not to say we don't do it simply because we have got opposition from that quarter. Tony [Blair] himself has said 'No favours, but fairness' and I think that is how we will approach it."
Liberal Democrat leader Charles Kennedy said Tony Blair had not made clear his plans for public sector reform.
"This has been the bedevilment of this issue through the second half of the election campaign and, I suspect, will carry on straight through the first half of the parliament just about before us," he told BBC Radio 4's World This Weekend.