Dean Creamer, head of Building Digital UK (BDUK), the government body overseeing the £1 billion Shared Rural Network project to improve rural coverage, said that officials had blocked the request to push back the initial deadline by 18 months.
Speaking in front of MPs on the Public Accounts Committee, he said, "That wasn’t accepted and we haven’t agreed that and we’ve said that Ofcom will make that assessment as expected."
Vodafone, Three and Virgin Media O2 wrote to government last year asking for an extension to the deadline, which requires operators to expand their coverage to reach 88 per cent of the UK's landmass by June.
Ofcom can fine companies up to 10 per cent of their global turnover if they fail to meet the targets, though officials said that any response would be "reasonable".
EE, which is contributing to the initial phase of the SRN project separately, reached its initial target earlier this year.
However, other mobile firms have struggled to build enough masts to plug the rural coverage gaps.
Government has set a target of delivering 4G coverage to 95 per cent of the UK landmass by the end of next year and to the entire country by early 2027.
In a report published in February, the National Audit Office said that the programme was running behind schedule and warned that high inflation had driven up costs.
Mr Creamer said, "In terms of the way the programme has been structured, there is clearly a big gap between that first licence obligation and then the target. So there is significant contingency, if you like, built into that, which is now being used."
Phase one of the SRN project is focused on 'partial not-spot' areas where at least one operator provides coverage. Phase two, which will be funded by government, will target 'total not-spot' areas, where no operators currently provide 4G services.
A Virgin Media O2 spokesman said, "We have already built more new sites than any other operator and remain committed to doing everything we can to deliver better mobile connectivity to rural areas."
A Vodafone spokesman said, "We remain committed to delivering on all elements of the SRN programme and have successfully introduced 4G to rural locations across the UK as part of the wider project which is due for delivery by Jan 2027. We have recently gone live with our 200th site bringing geographic 4G coverage to 83 per cent of the UK, on track to deliver the Government’s 95 per cent SRN coverage target by December 2025."
A government spokesman said, "Mobile network operators are responsible for delivering the first part of the Shared Rural Network and legally bound to reach 88 per cent geographic coverage by June 2024.
"The Shared Rural Network has already delivered substantial improvements to mobile coverage across the UK with 4G geographic coverage now at 93.2 per cent."