A new £310m factory in Hull that makes wind turbine blades has been hailed by ministers as proof that manufacturing has a “glittering” future in the UK.

The first batch of 75-metre blades have emerged from the plant, part of a vast “green port” built by Siemens and partners at docks that used to export Yorkshire coal.

The facility, which started operating in September, is due to employ 1,000 by the new year, up from 700.

The investment is considered a huge boost for a port area that had declined into a wasteland and a city with one of the UK’s highest unemployment rates.

Stephen Brady, the city council’s leader, said the plant, combined with Hull being named city of culture for 2017, was a positive “perfect storm”.

“This place is proof that manufacturing in this country and in this great city has a glittering, hi-tech future,” said the business and energy secretary, Greg Clark, at the site’s inauguration on Thursday.

Everything about Alexandra Docks is big, from the 90-metre turbine towers that stand waiting to be floated out in January to a windfarm off the coast of Cromer, Norfolk, to the blade factory itself, which spans an area the size of seven football pitches.

Inside its cavernous halls sit the 28-tonne blades under construction, made of balsa wood and fibreglass. Hundreds will be built there each year, destined for bigger, more powerful offshore windfarms in deeper waters up and down the UK’s coast.

Surprisingly, the manufacturing process is almost entirely done by hand, rather than robots. The workforce includes former supermarket workers, aerospace industry experts on second careers and builders who learned fibreglass skills locally from fitting bathrooms and making caravan parts. The blades are precisely moulded, finely finished and then painted.

“It’s a million miles away from a supermarket,” said Fiona Wright, 25, who used to work at Asda while studying biomedical science, and now makes the basic structure of the blades atop a mould.

Like many of the new employees, she was sent for several weeks to a blade factory in Aalborg, Denmark, for training. “I loved it. It was a great experience,” she said of her trip.

Ian Deer, 48, used to work at British Aerospace in nearby Brough, where Harrier jump jets were built. “It’s a different product but the skills are still the same. It’s all hand skills, reading drawings, reading instructions,” said Deer, a Siemens group leader with expertise in assembly work in metal, wood and fibreglass. He called the new facility “phenomenal” for the city.

Siemens said 97% of staff live within a 30-mile radius of the site, in a riposte to complaints in previous years that the UK wind industry was benefiting only foreign workers.

“For all those people that say there is this fear that we have German and Danish people taking away jobs for British people, this has been a perfect example of German and Danish people creating 700 jobs here in the UK,” said Juergen Maier, chief executive of Siemens UK.

For the next two to three years, the blades built at the site will be shipped to offshore windfarms around planned or under way in the UK, which has more offshore wind power than any country in the world.

But the blades from the Hull facility were originally intended for export too, for burgeoning markets in the Netherlands and Belgium. “We are not yet looking at exporting but of course as we get more successful and bring the cost down of the technology [we hope we will export],” said Maier.

The full order book for UK windfarms will initially insulate the plant from Brexit, but Maier said it was important that that “punitive” trade tariffs were not imposed as the UK left the EU. “If there are tariffs, it is not particularly helpful.”

Clark echoed those sentiments. “I want this plant to export,” he said. “The ability to trade without tariffs is very important.”

There is also concern in the industry over what happens if the pipeline of British windfarms begins to dry up in coming years.

Due to an overspend in recent years for renewable energy subsidies, partly because existing offshore windfarms were more productive than expected, there is little public money left for new projects in the run-up to 2020.

Clarity on support into the next decade was due in the autumn statement, but to the disappointment of many it was bumped to the 2017 budget.

However, Clark said investors should be reassured by the government’s actions since he was appointed in July, and what he called the administration’s “zeal” for clean energy.

“There’s no disguising, no ambiguity whatever, about the centrality of clean energy that we see not just to our commitments on climate change, vital though they are, and the energy sector, but to industry more generally,” he said.