Labour said the Royal Institution of Chartered Surveyors (RICS) was looking at an "appropriate benchmark" such as the average rents in an area or inflation. But RICS said it had no such proposals and it would not recommend ministers introduce a cap.
The Tories described the RICS comments as "embarrassing" for Labour. Mr Miliband unveiled the plans yesterday as part of Labour's European elections campaign.
Labour is keen to appeal to so-called "generation rent", the growing number of people locked out of the housing market because they struggle to get a mortgage. Under the plans, three-year fixed-rent tenancies would become standard. Letting fees would be abolished and landlords could only serve renters with two months' notice to leave with "good reason".
Jeremy Blackburn, RICS' head of policy and parliamentary affairs, said: "It is always important to consider all options which could potentially expand the supply of private rented homes, and to explore any that might make a positive impact on the sector.
"However, RICS is not developing proposals on rent benchmarks for the private rented sector, and we do not recommend that a government introduce a ceiling on rent increases. Labour is right to talk about 'generation rent', but arbitrary caps are not a solution."
Tory chairman Mr Shapps said: "Evidence from Britain and around the world conclusively demonstrates that rent controls lead to poorer quality accommodation, fewer homes being rented and ultimately higher rents - hurting those most in need."
Labour denied the plans amounted to rent controls and pointed to comments from Communities Secretary Eric Pickles supporting inflation-linked rent review clauses.