By Andrew Verity BBC personal finance reporter

A leading credit card provider has claimed consumers are being duped by low interest rates which feature prominently in advertising.

Egg, the online telephone bank controlled by Prudential, says many of the rates will, in reality, be offered only to a tiny minority of customers.

The bank carried out a mystery shopping exercise to find out how many customers were offered cheap interest rates on loans and credit cards advertised by its rivals.

Only one in ten of its mystery shoppers were offered the lowest advertised rates.

In the case of Barclaycard, only one in 25 were offered its best rate while Lombard Direct offered no one the lowest rate.

Consumers 'duped'

Mark Nancarrow, chief executive of Egg UK, said: "How many consumers would buy a hi-fi from a shop with a price tag reading 'Price agreed after purchase'?

We believe this is yet another example of consumers being duped by UK banks Mark Nancarrow, chief executive of Egg UK

"We believe this is yet another example of consumers being duped by UK banks - this time by dazzling headline rates which often fail to sparkle."

Egg said the banks were flouting the spirit, if not the letter, of advertising rules drawn up by the Office of Fair Trading.

The rules require credit card and personal loan providers to give the most prominent place in their advertising to their most typical rate.

Card research

Egg commissioned the respected research group NOP to carry out its research, with 200 researchers applying for credit by telephone - 50 for each of four lenders: Barclaycard, Capital One, Lombard Direct and Abbey National.

% offered lowest rate Barclaycard: 4% Capital One 'No Hassle Platinum': 32% Lombard Direct Personal Loan: 0% Abbey National: 4% NOP/Egg

At Capital One, 16 out of 50 were offered the lowest advertised rate for the "No Hassle Platinum" credit card of 11.5%.

But at Lombard Direct, not one of the 50 applicants was offered the lowest advertised rate of 6.9%.

And at Abbey National, only two out of 50 were offered the cheapest rate of 6.6% on the Abbey loan personal loan, available over the internet (see table).

We make it very clear what our typical rate is (17.9 per cent) Barclaycard

For Abbey National and Lombard Direct, the average rates offered to researchers were slightly higher - less than two percentage points - than the lowest advertised rates.

With Barclaycard, researchers were offered an average rate more than 6 percentage points higher.

Bank denials

The banks targeted by Egg flatly deny that and accuse Egg of drawing misleading conclusions.

A spokesman for Barclaycard said: "Some of Egg's points are fair enough - like saying it should be clear that whether you get the best rate depends on your circumstances.

"But we don't think they apply to us. We make it very clear what our typical rate is (17.9%)."

Lombard Direct and Abbey National said Egg's research was contradicted by their own internal monitoring of a much bigger sample of customers.

"The big problem is that the advertising rules are just too loose Mark Maguire, Egg

They were required by the OFT's rules to highlight their typical rate - meaning the rate with more customers than any other rate.

Abbey National said 70% of its customers last month were offered the typical rate (6.9%) or lower.

Lombard Direct said a majority were offered its typical telephone rate of 7.9%. Egg had focused on its internet rate of 6.9%, which was not available to telephone applicants.

Egg countered that customers could look at Lombard Direct's website - and go through several stages of the application process - without being told the rate on its front page was only available over the internet.

"The big problem is that the advertising rules are just too loose.