NEW YORK (TheStreet) -- The future of startup technology companies in London is in question following the U.K. referendum to leave the European Union.

"Uncertainty" is the word that Julie Sandler, Principal at Madrona Venture Group, used to describe the environment in London, one of the world's largest technology development cities.

"Anyone steeped in startup dynamics will tell you that two of the most important ingredients to a startup community are capital and talent," Sandler said on CNBC's "Squawk Box." "As of exactly one week ago, the perception around both of those issues was called into question."

The question of where up-and-coming European specialists should take their talents has changed. Putting herself in their shoes, Sandler says "I'm thinking, 'where do I want to build my career in innovative companies?'"

"Up until last Thursday, almost unquestionably London might have been the top of my list for the capital reasons ... [but] that has been called into question," Sandler noted.

Paris, Berlin and Amsterdam are now alternative options according to Sandler, who believes that "the reality is tech companies like to be near their customers." For the U.K.-based entrepreneur, the question is now how to continue to attract talent to London. Sandler believes there are two questions those business owners must think about for future success.

"One, how am I going to advocate policy makers around empowering movement of people with specialized skills that are in demand? And two, what am I going to do to attract the machine learning specialist from Helsinki to make sure that he or she feels compelled to join my company," Sandler said.

In a broad warning to entrepreneurs that "it's very important to be aware of broader market trends" such as "the opening and closure of foreign markets," Sandler did provide optimism through noting that startups "also get built during down cycles." Citing the recent example of Twilio's (TWLO) - Get Report successful run from spring of 2008 founding to IPO on the day of the Brexit, Sandler noted that "they capitalized on a bunch of important themes that emerged out of that down cycle."

"When you make predictions or blanket statements about the U.K., you have to exercise a measure of restraint," Sandler added.