Google is rolling out its customised search function to all of the search engine's users, affecting anyone involved in SEO.

Previously only Google users who had signed-in and had enabled their web history on their account have found their search results customised. Now all users will find that if they favour a certain site Google will feature it higher in its search results.

Google says the aim of the move is to provide better search results to its customers. It will also be able to target ads more precisely by building a profile of its users web activity.

As search results will vary from user to user it will make it harder for SEO specialists and marketers to convince Google's algorithms to push a site higher up its search ranking.

A website's position in a Google results page will also be harder to analyse as it will vary from user to user.

"Now when you search using Google, we will be able to better provide you with the most relevant results possible", said software engineer Bryan Horling and product manager Matthew Kulick in a blogpost announcing the change.

Users can turn the personalisation off it remains an opt-out rather than an opt-in service, alarming privacy campaigners.