The bosses of 30 European start-ups have called on politicians to reform rules around employee stock ownership to help them attract more talent and compete with their Silicon Valley counterparts.

Chief executives from firms including TransferWise, Stripe, PayPal-owned iZettle and recently listed Farfetch co-signed an open letter urging lawmakers in the continent make changes to existing regulations on employee stock options.

"Without delay, we call on legislators to fix the patchy, inconsistent and often punitive rules that govern employee ownership," they said in the letter Wednesday.

Employee stock options grant employees the ability to buy a specified number of shares of the company they work for at a certain price and at an agreed time.

According to U.S.-based venture capital firm Index Ventures, tech workers in the U.S. own twice as much equity in the companies they work for than their European counterparts.

The investment firm adds that there is an imbalance country-by-country, with the U.K. having the "most favourable" regulatory and tax conditions for start-ups when it comes to stock options and German and Spain being "most in need of change."