Barclays is making 10 per cent extra from online promotions by building a profile of each web site visitor and tailoring advertising accordingly.

But experts have raised concerns over issues of privacy.

The bank aims to maximise conversion rates ­ the number of sales made online compared with the amount of visitors ­ by analysing behaviour on and off the Barclays site, said head of online marketing and content Julian Brewer.

“It is not just what people do on our web site, it is what they arrive with,” he said.

The system analyses the IP address of a PC to find out where it is located, what site a user has come from, and what search term was entered to get to the Barclays page. It can also see what browser is being used.

The data is then compared with the behaviour of previous visitors so advertisements and promotions displayed on the front page can be tailored according to what is most likely to sell.

A number of high-street banks use similar systems already, but have been wary of publicity.

Barclays is keen to allay privacy concerns. Visitors to the site do not have the option of turning the system off, but it does have built-in anonymity to protect privacy, said Brewer.

But experts say such systems still breach guidelines.

It is always controversial to be presented with different services depending on your history, said London School of Economics research fellow Gus Hosein.

“Many businesses are steering clear of this because they do not want to inflame privacy debates,” he said.

The system, called TouchClarity, comes from vendor Omniture.