Plans for the way farming subsidies will be dealt with after Brexit have been set out by Michael Gove.
Farmers will receive payments for "public goods", such as access to the countryside and planting meadows.
The environment secretary told farmers the government would guarantee subsidies at the current EU level until the 2022 election. There would then be a "transitional period" in England.
The National Farmers Union said it was time for "a new deal" for the UK.
Fergus Ewing, the Scottish rural economy secretary, said Mr Gove had left "too many questions unanswered".
Meanwhile, a report warns Brexit trade deals could threaten UK food security.
MPs and peers in the All-Party Parliamentary Group on Agroecology (APPGA) say ministers must ensure farmers are not undermined by future trade deals which permit imports of food produced with lower welfare or environmental standards.
Mr Gove, who has promised that standards will not be compromised after Brexit, addressed two farmers' conferences in Oxford on Thursday.
His speeches came ahead of the government's agriculture plans being published this spring.
The current payment system - £3bn a year to UK farmers - is based on the amount of land farmers own.
Detailing how the European Union's Common Agricultural Policy (CAP) will be replaced after Brexit, Mr Gove said the CAP was "unjust" and "doesn't really reward efficiency".
The government has agreed to maintain current subsidies for three years after Brexit, until 2022, and Mr Gove said the payments could continue until 2024 but the length of time would be down to "consultation".
Mr Gove said during that time he aimed to reduce the largest subsidies, with a maximum cap or a sliding scale of reductions.
He said there should be a "smooth path" towards a new way of paying farmers when EU subsidies ended and that a new method would "use public money for public goods".
The plans would see farmers rewarded for planting woodland, boosting wildlife, improving water quality and recreating wildflower meadows.
Speech 'no comfort'
The environment secretary said he was "confident" about the future of British farming and that Brexit would allow the UK to "leverage" the advantages of Britain producing "the best food in the world" and "some of the most innovative farmers in the world".
New trade deals with other countries outside the EU would provide new markets for the "superb food" Britain's farmers produce, Mr Gove said.
Lib Dem leader Vince Cable said although farmers might take "some reassurance" from the announcement of a transition period, the end of that "could result in a lot of turbulence and a lot distress".
Minette Batters, deputy president of the National Farmers Union, welcomed incentives to protect the environment but said her key concern was over future trade deals.
She said: "We're very proud of our high standards of environmental protection, of welfare, in the UK and we want those to be respected in any trade negotiation and we do not want to see cheaper food produced to lower standards."
Mr Gove's counterpart in Scotland, Mr Ewing, said Mr Gove's speech "leaves far too many questions unanswered for any comfort to be taken".
He said it did "not cover a whole variety of vital support schemes", such as environmental programmes, "which are crucial to ensure the continued economic well-being of all of Scotland's rural communities".
By Roger Harrabin, environment analyst
It has driven birds out of the countryside, led to soil erosion, and caused the loss of woodlands and wildflower meadows.
Over many years, attempts to reform the policy have been resisted by farm unions, especially in France.
But Brexit has given Mr Gove the opportunity to produce a farm policy made in Britain.
His changes will alarm those farmers who will need to change their whole business model to get subsidies after Brexit.
It will be welcomed by some efficient modern farmers who have already accepted that being paid by the public for owning land can't be justified.
The changes will impact on the countryside and food production.
It's too soon to tell exactly how.
'Best possible deal'
Mr Gove's speech comes as the APPGA report says post-Brexit trade deals could pose the biggest peacetime threat to the UK's food security.
According to the group, the import of cheaper foods that are produced to lower safety and welfare standards could place UK farmers at a disadvantage.
"To compete with these lower prices, domestic farmers could seek to tighten their margins and therefore cut corners with regards to environmental regulations," the AAPG said.
"If the UK is unable to protect its farmers from being undermined by lower welfare imports, farmers are likely to resist improvements and may even press for UK standards to be lowered."