A stiff email is heading to your inbox if you are one of Netflix's four million UK account holders who have been a bit naughty with the streaming giant's T&Cs.

Having turned a blind eye to subscribers sharing their password with anyone who isn't at your address, the change will spell a significant tightening up of Netflix's UK operation.

The email being sent out to four of the 15 million account holders who have been identified as rulebreakers warns: "Your Netflix account is for you and the people you live with – your household. You can easily watch Netflix on the go and when you travel – either on your personal devices or a TV at a hotel or holiday home."

The crackdown employs software that has trawled through the usage patterns of 15 million UK subscribers. Globally, it is now estimated that about 100m homes pay for the viewing service while also sharing their passwords with friends and family.

As a remedy, Netflix has set up a "paid sharing" option, where extra users can be added for £4.99 a month.

Netflix says it is getting tougher in a bid to boost profits after new subscriber growth rates slowed down and the policy has already been rolled out in Latin America, New Zealand, Spain, Portugal and Canada.

The company has meanwhile advised Sky, Virgin Media and BT to prepare for a potential customer backlash from customers who view its content through their services.

Netflix also offers a no-frills basic £4.99 a month option, which includes advertising for the first time.

The move raises the question of whether passcode sharing is actually illegal. And the short answer is yes: the Copyright Designs and Patents Act 1988 is the main legislation dealing with UK copyright and under its terms the Intellectual Property Office reports that password sharing is a breach of copyright law.