Skype has alerted a number of its "SkypeIn" users that they will lose the use of their London telephone numbers as of next month.

SkypIn is a service allowing calls made to particular landlines to be re-routed to a user's Skype account. Customers pay a quarterly or yearly fee to Skype for the use of a particular landline number, which Skype leases from various telecoms providers.

In a message on Wednesday, however, Skype alerted users that their central London telephone numbers will no longer be valid as of 20 December.

"Unfortunately, we have to return some of the 0207 SkypeIn numbers to one of our suppliers of London numbers," Skype said in the message. "This means your number will stop working from December 20th, 2007. We realise the inconvenience this will cause you, and sincerely apologise."

The company is giving users credit for a year's subscription to a new telephone number, and has created a "help" page explaining how to activate a new SkypeIn number.

Customers have until 20 December to activate a new number.

When tested the process, central London numbers - beginning "0207" - were no longer available.

"Due to changes in the capital's number plan there is a limited availability of 0207 numbers and we cannot guarantee that you will find a 0207 number to replace your existing one," Skype explained in the message. "We have plenty of 0208 numbers and 0203 numbers which OFCOM has introduced to cover the whole of Greater London."

The change means a serious inconvenience for those using SkypeIn for business purposes, as they will be forced to inform customers and associates of the change within a month.