A huge solar farm in Lincolnshire and another in Cornwall started generating green electricity on Thursday to become the UK's two biggest solar installations, as developers rushed to beat an imminent cut in government subsidies.

The 1MW Fen Farm solar park and the 1.4MW Wheal Jane park in Truro are two of several such large-scale projects rushing to connect to the grid. They are trying to benefit from a higher level of feed-in tariff payments before the government cuts the rates by up to 75% on 1 August.

When the cuts were confirmed last month, ministers defended them on the grounds that the funding for payments needed to be protected for householders. But energy industry figures and campaigners warned that making such large projects financially unviable would "crush" the solar industry and cost the UK "major manufacturing opportunities, jobs and global competitiveness".

The developers of the Truro park on the site of a disused tin mine worked around the clock to finish the project in time to beat the tariff cuts. Solarcentury and Lightsource Renewables originally planned to finish the park by the end of August. They were forced to bring both projects forward, at significant extra cost.

Although the solar park in Lincolnshire was always planned for completion before August, its developers, Ecotricity, experienced similar concerns about not completing the project in time. Founder Dale Vince, said that connecting to the grid by 1 August is an insecure business. "When you think that we're finishing three weeks ahead of the deadline, we're cutting it fine enough for my liking," he said. "You've got a situation where the grid companies aren't that amenable. They don't get anything out of accommodating renewable energy on the grids so it's never an easy thing. It's a bureaucratic process and more often than not there will be a delay."

Not all the projects are fortunate enough to have even three weeks' leeway. Silicon Vineyards, the developer of a solar park at Benbole Farm in Cornwall, is working right up to the deadline.

Despite the cut in payments – from 30.7p per Kwh currently to 8.5p from August for field-size installations over 250Kw - some companies are pushing ahead with large-scale projects that will miss the deadline.

They are hoping that the government will announce in mid-July that renewable sources of energy including solar are now eligible for a higher level of a separate subsidy scheme, Renewable Obligation Certificates (ROC).

Hopes in the industry are also being pinned on the exploitation of a legal loophole in the feed-in tariff system. Ray Noble, solar specialist at the Renewable Energy Association, said companies are putting plans on hold while they seek legal advice and wait for Ofgem to indicate whether smaller solar power projects could later be increased in size and still retain the higher rates.

"There'll be some innovative ways of trying to make it work," Noble said. "Most of the developers have kept very quiet while they lobby government to say they're losing lots of money and that the system needs to be changed for them to make any plans."

Ecotricity's Vince was critical of the government's support for solar power. "Our government has got its eyes focused on big nuclear and clean coal, and solar has become an inconvenient success story. But it seems crazy to me to have a cap. We need large scale solar to make the UK more energy independent and reduce our carbon. Large-scale solar is critical if this government is serious about being the greenest ever."