A statement from Sony's UK division confirms the price increase, but does not mention the new numbers customer will have to get used to:
"We are dedicated to bringing PlayStation Plus members the best possible service with the most compelling content, from 1st September we will be increasing the price of monthly and three month PlayStation Plus subscriptions in line with market conditions. We will continue to invest in PlayStation Plus to ensure an unparalleled experience, featuring the best quality games and features."
However, users have been getting emails regarding the change, suggesting that three month memberships will increase from £11.99 to £14.99, as seen here. As of now, it seems like the rest of the world will be unaffected, but we will have to wait for more information from Sony to be sure. With no other explanation than the need to adjust to the market, it may look like the company is attempting to push gamers into the 1 year subscription. Regular outages, cloud storage issues, and slow download speeds are common problems reported by PlayStation Plus users, so whether or not is is a good idea to raise the price without announcing a fix or major improvement, remains to be seen.
Source: Push Square