The association of Teachers and Lecturers has warned a parliamentary select committee that "you will get what you pay for" in the wake of Ofsted's cuts to its inspection costs.
The schools watchdog last week announced that it had slashed costs by almost halving the number of contractors it works with, from five to three.
Martin Johnson, ATL's deputy general secretary, told a Children, Schools and Families select committee hearing that Ofsted's inspection quality could suffer as it tries to reduce costs.
Afterwards, Mr Johnson said: "This is something I feel particularly strongly about.
"Ofsted's retendering of its inspection contracts is an opportunity for it to make savings. If that is the case, then it will likely have to reduce fees.
"It might be the same inspector carrying on for lower fees, but it seems likely that an inspector would go elsewhere to stay on the same pay.
"This means other people with less experience - in breadth and in vision - will replace them. So, in essence, you will get what you pay for."
The union called for an end to Ofsted inspections, adding that all inspections should be carried out at local level to integrate inspectors and improve schools by working more closely with them.
The Association of Professionals in Education and Children's Trusts, the inspectors' union, believes inspectors will simply adjust to the changes brought in by the new contracts.
John Chowcat, the general secretary, said: "Inspectors have adapted to the Section 5 inspections, which rely on more self-evaluation and have been capable of seeing through any outlandish claims that might come from a school."
He added: "Education inspectors generally don't work at the higher end of the pay scale, anyway. They have never been best paid."
Ofsted has been trying to cut its costs since 2005, and had hoped to reduce its overheads by 30 per cent - around Pounds 80 million - by 2009.
However, it is understood that the cost-reduction figure is nearer Pounds 60 million. It hopes the new inspections contracts will provide further "significant" savings.
The three contractors are CfBT Education Trust, which will cover the north of England, Serco Group, which will operate in the Midlands, and Tribal Group, which will work across the South.
After the announcement, Christine Gilbert, chief schools inspector, said: "Ofsted has a track record of delivering high-quality inspections through partnership with contractors. These new arrangements will ensure Ofsted has a high-quality, flexible and diverse workforce helping us deliver consistency and value for money in the way we inspect."
An Ofsted spokeswoman added: "Throughout the evaluation process the focus was on delivering high quality along with considerations of value for money for Ofsted and for the taxpayer.
"Evaluation included a quality threshold, which meant that a bid that did not deliver acceptable quality would not be awarded a contract."