On a sun-soaked street in central Leipzig, a union jack flag flies at half-mast.

Its owner, Dr Sonja Weingarten, runs Der Englandladen, a shop that sells a huge range of British products - from Marmite to Mars bars - to the large local expat community.

"They are asking us whether our shop will still be here," says Dr Weingarten, a committed anglophile.

"All of them thought the 'remainers' would win."

Foreign investment has been vital in transforming this city, once part of the former German Democratic Republic (GDR), and now often referred to as the "boomtown of eastern Germany".

In recent years, business behemoths such as Amazon and Porsche have established bases in Leipzig, occupying rows of warehouses that were derelict and decaying.

Image copyright Getty Images Image caption Amazon is just one of the businesses that have moved some operations to Leipzig in recent years

Highly skilled workers from across Europe followed in their wake, including Philip Rooke, the British chief executive of Spreadshirt, a thriving e-commerce firm that was founded in the city, and which now has offices and production facilities in Poland and the Czech Republic, as well as in the US.

"I'm slightly devastated," he says, trying to control what are obviously deep emotions.

Open borders

"We have 22 nationalities working in our office here - it's a very multicultural world - and I feel my home country cutting itself off from that will lose all of those values."

Having access to the EU's open borders has been vital for Spreadshirt, which last year shipped 3.6 million made-to-order T-shirts, mugs and hoodies to more than 180 countries.

For smaller countries outside the EU's economic bloc, Spreadshirt has been forced to offer a reduced service.

Image copyright Spreadshirt Image caption "I see the benefits of Europe every day here in Leipzig," says Spreadshirt's chief executive Philip Rooke

"Where we don't have open borders, with Norway and Switzerland, it gets very difficult," says Mr Rooke, "and we end up doing a poor service, and we end up making fewer sales."

The same may soon apply to the UK, he warns.

"This kind of global sales environment would not be possible without open borders. The complexity if we don't end up with open borders, or we end up with the Norwegian system, will go up, and therefore we'll put less effort into the UK."

Mr Rooke, who hails from the English county of Wiltshire, spent many years building up e-commerce companies back in Britain, and is worried about the sector's future.

"There are many of the people I know in the investment community, who are not considering UK e-commerce businesses now, because they would prefer to invest in European ones that can reach a much larger audience in a much easier way. That hurts."

Image copyright Spreadshirt Image caption Spreadshirt says having open borders across Europe is key for its business

Can he see himself ever returning to the UK?

"Possibly not," he says, after a thoughtful pause.

'People haven't given up'

Jesse Wragg, a fellow Brit working in Leipzig, is slightly more optimistic. UK nationals working in Germany have been hit by the vote, he says, but he's hopeful that the two countries will find a way to maintain freedom of movement.

His clients, 60% of whom are based in Britain, "didn't want to talk" on the morning after the Brexit decision, but on Tuesday, it was back to "business as usual", says Mr Wragg.

"People haven't given up on Europe as a whole," he says.

Image copyright Thinkstock Image caption Foreign investment has been vital in transforming Leipzig

The impact of Brexit is not confined to businesses in the area with British staff and clients.

Birgit Stodtko, the international director of the Chamber of Commerce and Industry for the southern region of the neighbouring state of Saxony-Anhalt - which represents more than 57,000 small and medium-sized companies (SMEs) - says local firms "are all worried" by the result.

Some €1.2bn (£986m) worth of goods were sold by SMEs in the region to the UK last year, including chemical and pharmaceutical products, food stuff and metals.

"Great Britain is number two in our export ranking," she says, "and in the last five years we've doubled our exports to the UK."

Now, Mrs Stodtko adds: "Everything will be very difficult, especially the bureaucracy, red tape and administrative burden."

Businesses also need Britons to come and work in the area, she says, as Saxony-Anhalt "needs specialists in every area of the economy".

'Disbelief and shock'

Yet there are strong indications that skilled UK workers might well continue to work in the country's commercial centres.

Carrie King, who works for job search company Jobspotting in Berlin, says there has been a clamour for information on how Brits can ensure their future in Germany.

Image copyright Carrie King Image caption "People are in limbo," says Jobspotting's Carrie King

Blog posts she wrote explaining British expats' options were read so widely on Friday, she says, that the volume of traffic almost crashed the firm's servers.

"The mood is one of disbelief and shock, people are in limbo - they are disappointed and are wondering what is going to happen in the coming months."

Back in Leipzig, Dr Weingarten is hoping Britons in this part of the world find a solution, if only for the sake of her English goods store.

"I hope that it will exist," she says.

"Otherwise, we'll change it into a Scottish or Irish shop."