Only one-fifth of British farmers have reliable mobile signals across their whole farm, a new survey by the NFU has found.
Farmers' ability to meet the challenges of the future are being held back by poor broadband and mobile connectivity, the union said.
Its latest digital technology survey has revealed that just 21% of farmers overall have reliable mobile signals across the farm.
Fewer than half of farmers reported broadband speeds which they say are adequate for their business.
And one in 20 farmers reported to the NFU that they have no reliable outdoor mobile signal on their farm at all.
Submitting VAT returns, GPS on tractors, talking to customers and more all rely on strong digital connectivity.
Consistent access to the internet is also needed for farmers so they can comply with UK regulations and operate their businesses efficiently.
But the NFU said current broadband and mobile connectivity across the UK's rural areas was 'unacceptable'.
To achieve better connectivity, the union is calling for the Shared Rural Network to remain a government priority and be finished by 2025.
Broadband schemes must also apply to all types of broadband access, not just fibre, the NFU said, and farmers need more support to access agricultural-specific digital skills training.
NFU vice president David Exwood said: “We need to produce more of what we do well here, this means being as efficient and productive as possible, and access to the internet is vital for businesses to do this.
“Even with the positive increase in access to superfast broadband for over a third of respondents, it is unacceptable that 4 out of 5 of farmers do not have reliable mobile signals throughout their farm.
"Not only does this impact the day-to-day running of rural businesses, but it is dangerous to leave a farmer with no way of communicating in a time of crisis."
Mr Exwood said the lack of internet access was preventing UK farmers and growers "from doing what they do best – producing homegrown, climate-friendly, and affordable food."
"This year’s survey shows that connectivity is only increasingly slowly, and the farming industry is still lagging significantly behind the rest of the country," he added.
“We will continue to campaign for investment in the country’s digital technology infrastructure - which is key to productive farming businesses."
The NFU's digital technology survey summarised a snapshot of rural connectivity from 814 farmers and growers, surveyed between between December to February.
