Amazon raised the monthly rate for its Prime service by 18% on Friday, from $10.99 to $12.99. The annual Prime membership stays at $99.

Amazon Video comes included in both the annual and monthly membership. The a la carte price for those who are not Prime members will remain the same, at $8.99 per month.

The most recent Prime subscription hike came in 2014 when the yearly membership went from $79 to $99.

Amazon gave no specific reason for the price increase, beyond a statement that it continues in its "unwavering" conviction that Prime is the best value in the history of shopping.

Forrester analyst Brendan Witcher said "when compared to similar offerings in the market, the $10.99 monthly Prime offering was priced too low." Cobbling together a similar offering including delivery and free Cloud storage would cost fare more, he said.

Not raising the price for annual subscribers is more about creating loyalty, he said. "Leaving the annual rate as is fits with their strategy to encourage customers to see Amazon as a brand that will be part of their regular lives, rather than a monthly purchasing consideration."

Prime membership is growing, though Amazon itself will only say that it has "millions" of customers.

Consumer Intelligence Research Partners estimated in October that Amazon had 90 million Prime members. They are highly lucrative for the company because free shipping and the other perks that come with membership mean Prime members spend more on the platform. Last year CIRP estimated that about 19% of Prime members pay monthly.

Earlier this month, CIRP released an analysis that found that Prime members spend $1,300 on average each year, compared with $700 for non-Prime customers.