Farmers' interests are best served by remaining in the European Union, the National Farmers' Union has said.
It passed a resolution following an "overwhelming" vote in favour of staying in the EU, which it said was based on the "balance of existing evidence".
The union - which has 55,000 members in England and Wales - announced its position after a vote by its council.
However, the NFU said it would not be actively campaigning in the referendum.
It said its council members looked at a number of issues, including the impact leaving the EU would have on agricultural trade and the availability of labour.
It has also completed 28 roadshows to debate key farming issues in the referendum with members, and sent two reports to its members.
The organisation said it was not joining any campaign groups and would not be telling its members how to vote.
The union has not released how each of its 90 council members voted.
However, its president, Meurig Raymond, said there had been an "overwhelming" vote in favour of staying in the EU.
"We believe it's for the betterment of the future of British agriculture," he said.
"On all the surveys we have done in the NFU, the majority believe that we should stay in."
He added: "We have had 28 meetings across the country in the last three weeks.
"We have spoken to three or four thousand people. So the delegates here today brought the views from those meetings to our council meeting."
By Claire Marshall, BBC environment correspondent
While the NFU doesn't represent all farmers, this decision will be influential.
As an established well-funded organisation, the NFU has access to information and expert advice that the average farmer doesn't have time to seek out.
The debate in the council chamber was impassioned, with members talking about what the decision could mean for the future of their children.
The judgement will also play an important part in the wider European debate. Over the last few weeks key figures from both sides of the referendum campaign have been on the phone to the NFU asking for updates, keen to know what side they will come down on.
Now, everyone knows what the council thinks - it's now down to the farmers to decide.
The NFU says more than 70% of full-time farmers in England and Wales are members.
The resolution comes after the NFU in Scotland and the Tenant Farmers' Association both expressed support for remaining in the EU.
However, farming minister George Eustice, who wants to leave the EU, said "virtually every problem that the NFU complain to me about is a direct consequence of dysfunctional EU law".
He said farmers who "want to see change and a better future" should vote to leave.
Last month, Prime Minister David Cameron, who is campaigning to remain in the EU, warned British agriculture would suffer if the UK votes to leave the union.
He said farmers could lose as much as £330m on lamb and beef exports if Britain were to leave.
However, former environment secretary Owen Paterson has said Britain's farmers would be better off outside the EU.
The Conservative MP said Britain would then be free to set its own subsidies and its own environment policies.