Some of the UK's most powerful tech entrepreneurs have warned David Cameron against clamping down on immigration

The UK's most powerful technology leaders are warning Prime Minister David Cameron that their businesses will suffer if he follows through with plans to reduce the number of skilled migrant workers entering the UK from outside Europe.

In an open letter, 235 of the nation's leading entrepreneurs and investors asked the Prime Minister to reconsider plans that will make it even harder for them to employ skilled workers outside the EU.

The people behind the letter, which was orchestrated by Coadec (The Coalition for a Digital Economy), hail from some of the UK's best-known technology companies including Google DeepMind, Citymapper, Shazam, and SwiftKey.

Autonomy founder and billionaire Mike Lynch also gave his signature, as did angel investor Sherry Coutu and sharing economy advocate Debbie Wosskow, both of whom have written reports for the government in the last year and advised it on how to support the UK's digital economy.

It's worth noting that around 20 of the signatories also signed the pre-election tech letter in The Guardian backing the Tories.

One of the most prominent names not to appear on the list of signatories this time round is venture capitalist and government advisor Eileen Burbidge. However, Burbidge has told Business Insider on previous occasions that immigration and allowing UK tech firms to hire skilled talent should be a number one priority for government.

"We absolutely need highly skilled talent," Burbidge said last month. "It's difficult because obviously there's a larger political narrative which is about the stress on social services of an ever-growing population, but I think that what we can do are small measures, small things about making sure that we know where certain pockets exist for need or demand, and how to service that."

Given that Burbidge is now chair of Tech City UK and therefore a government spokesperson, it's likely that she felt she was unable to support a government lobbying effort. However, Business Insider understands that she will be doing all she can in her role as Tech City UK chair and UK fintech envoy to ensure the government doesn't clamp down on skilled immigration.

Following the general election, Cameron's Tory government revealed that it planned to "significantly reduce" the number of skilled workers coming to Britain from outside the EU. He said this was necessary as net migration levels were at record levels.

The letter comes as the government's Migration Advisory Committee consults how the UK can further tighten rules around the entrepreneur visa, which allows those outside the EU to come to Britain and start a business, and the Tier 2 visa, which enables UK technology companies to hire people from outside the EU and bridge skills gaps.

Companies like Google and Facebook rely on the Tier 2 visa to bring staff from their US offices to their London offices.

The government is exploring whether it should raise the minimum salary threshold on the Tier 2 visa so that it only applies to people working at director-level roles.

Sarah Wood, CEO and cofounder of Unruly, said: "If the government is serious about growing Britain's digital economy and competing on a global stage, it cannot afford to cut off the oxygen that feeds our ecosystem - skilled talent from around the world, drawn to London and the UK precisely because of its diversity, creativity and vitality.

"We simply could not have scaled the business to the size it is today (15 offices, £28.7m revenue) without the programming expertise and language skills of international software engineers and digital professionals from around the world."

Tech City UK estimates that 1.46 million people work in the UK's digital economy, which represents 10% of the UK's GDP, and is the biggest and fastest-growing of the G20, set to hit 12% of GDP next year.

"The UK has become a global tech hub thanks in large part to startup founders, investors and employees from across the globe, including many of us who were not born in Britain but choose to invest our time and talents here," the letter reads.

"We call on you to ensure that any future changes to the immigration system make it easier, not harder, for qualified digital entrepreneurs to come to the UK to start their business, and for growing startups to hire top international talent."

Guy Levin, executive director of Coadec and a former political advisor to the Treasury, said any changes to the immigration system must make it easier, not harder, for digital entrepreneurs to come to the UK to start their business and for growing startups to hire top international talent.

A 2013 O2 report, The Future Digital Skills Needs of the UK Economy, estimated that 745,000 additional workers with digital skills would be needed to meet rising demand from employers between 2013 and 2017. Halfway through that time frame companies are still crying out for talented workers with certain skills.

The letter acknowledges that the government has taken a number of steps to support the UK's digital economy, including giving tax breaks to investors that back startups through the Seed Enterprise Investment Scheme (SEIS), and its support for the sharing economy.

The government also relaxed the rules around the Tier 1 Exceptional Talent visa, allowing companies to hire groups of five people at once and opening it up to those that show "exceptional promise."

Here's the letter itself:

Dear Prime Minister,

We represent a cross-section of the UK's digital startup and scale-up ecosystem, including the founders of Citymapper, DeepMind, JustPark and SwiftKey. The UK has the largest and fastest growing digital economy in the G20, worth over 10pc of GDP.

During the election campaign you argued that the UK should be 'the startup nation in Europe, and one of the great startup nations in the world'. We share this ambition and applaud your government's long-standing support for the UK's tech community. From SEIS to support for FinTech and the sharing economy, you have championed entrepreneurs and innovators in the interest of securing our country's long-term economic growth.

However, finding talent with the right skills and experience we need to grow our businesses remains one of the biggest barriers to achieving that ambition. The UK has become a global tech hub thanks in large part to startup founders, investors and employees from across the globe, including many of us who were not born in Britain but choose to invest our time and talents here. We are very concerned that changes to immigration policy will make it more difficult to attract and recruit the talent high-growth companies need to compete and succeed in a global marketplace.

The government's Migration Advisory Committee is currently examining proposals aiming to further restrict the Tier 2 system of skilled work visas and to reform the Entrepreneur Visa. Further restrictions on skilled migration could restrict the growth of our businesses and hurt the UK's digital economy.

We call on you to ensure that any future changes to the immigration system make it easier, not harder, for qualified digital entrepreneurs to come to the UK to start their business, and for growing startups to hire top international talent.

It is of course also vital that we continue to support the growth of digital skills within the UK, and we stand ready to do our part.

Sincerely yours,