Both parties say food diversity must be protected
The Conservatives have pledged to introduce an ombudsman to help settle rows between farmers and supermarkets.
Frontbencher Nick Herbert accused the government of "dithering" on the issue of an ombudsman - which was recommended by the Competition Commission.
He said a Tory government would create one to "curb abuses of power which undermine our farmers".
The government said it was looking at how best to enforce a new code of practice to govern the relationships.
Both the Conservatives and the government have been outlining their plans for British agriculture at the Oxford Farming Conference.
Shadow environment secretary Nick Herbert called for a "new age of agriculture" where farming was important in responding to a rising population and climate change.
He accused the government of "letting down British farmers" and criticised it for having failed to set up an ombudsman to govern disputes between retailers and suppliers.
He said a voluntary code of practice governing the relationship was not "worth the paper it is written on" unless it was properly enforced.
"It is not enough to talk loosely about a fair market or the need for better labelling," he said.
"We need action, with a supermarket ombudsman and legislation to enforce honest labelling if the retailers won't act."
He told the BBC an ombudsman could put a stop to practices like "retrospective discounting", where supermarkets go back to farmers and say they want to drive down prices which have already been agreed.
Following a long-running inquiry, the Competition Commission concluded in April 2008 that the supermarket industry was operating in the public interest and leading retailers such as Tesco were not overly dominant.
But they called for an ombudsman to resolve disputes and ensure retailers did not exploit relationships with suppliers to push through unfair or retrospective contractual agreements.
HAVE YOUR SAY
Let's go back to rationing - people were less glib about wasting food when they didn't have any!
Margaret Woodward, London
Mr Herbert said supermarkets had got "too much power" which could mean smaller farms were damaged and did not invest, which could drive up prices in the long term.
He said: "While the government dithers the Conservatives are clear: we will introduce an ombudsman to curb abuses of power which undermine our farmers and act against the long-term interest of consumers."
For the government, Environment Secretary Hilary Benn told the BBC Radio's 4 Today programme: "We are looking at the Competition Commission's recommendations. It is a very important report that they have produced.
"We have already agreed there is going to be a new code of practice to govern these kind of relationships, which will come in next month, and we are considering how best we can enforce that."
National Farmers' Union president Peter Kendall said: "While we await further details of the Conservatives' proposal with interest, we would look forward to working with any government that sees the creation of a grocery market ombudsman as a priority in delivering the Competition Commission's recommendations in full."
Liberal Democrat environment spokesman Tim Farron said the government had "colluded in the dominance of the supermarkets" by not introducing an independent regulator.
But he said the Conservatives wanted to sound tough but were "fighting a gun battle with a water pistol".
"Farmers will only get a fair deal if a new regulator is given the teeth to enforce a strict, legally binding code governing the relationship between supermarkets, suppliers, and pricing," he said.