It's not often that Silicon Valley rivals join forces for something, but tech companies tend to stand together when it comes to governments using their products to spy on its citizens. And there are few proposals more intrusive than those found in the UK's draft Investigatory Powers Bill, which has prompted Facebook, Google, Microsoft, Twitter and Yahoo to make a joint submission expressing their concerns over this so-called snoopers charter.

The five companies are part of the Reform Government Surveillance coalition, which aims to address and reform current and potential laws regarding government surveillance.

The firms have warned the UK government that the Bill "could have far-reaching implications - for our customers, for your own citizens, and for the future of the global technology industry." In December, Apple voiced its protest at the proposals, saying: "We believe it would be wrong to weaken security for hundreds of millions of law-abiding customers so that it will also be weaker for the very few who pose a threat."

In today's submission, the companies also warned that the complex wording of the Bill could mean that firms are forced to weaken their encryption or provide backdoors, despite promises from the government that this wouldn't happen.

The bill also suggests companies may be forced to generate new sets of customer data simply because they're required to retain it. Part of the proposed legislation would require tech firms to store users' data for up to twelve months, including a record of every internet site visited, and allow government agencies unfettered access to the information; a form of online monitoring banned in the US, Canada, and every other European nation.

The US firms also state that the bill presents a risk to UK employees who work for the companies, as they may be targetted by law enforcement. "We have collective experience around the world of personnel who have nothing to do with the data sought being arrested or intimidated in an attempt to force an overseas corporation to disclose user information."

UK telecommunication companies Vodafone and EE have joined in the criticism of the bill, saying it could "significantly undermine trust in the United Kingdom's communications service providers."

Two weeks ago, it was reported that the UK government wants a note added to the bill that would see the bosses of any tech company that warns its users that British agencies are monitoring them face up to two years in prison.