• Michael Ballack will not make a decision until after World Cup • Possible retirement could further weaken Chelsea after transfer ban

Michael Ballack has said he may retire after next summer's World Cup, giving Chelsea further cause for concern as they come to terms with their transfer ban.

Ballack's £120,000-a-week contract expires next summer and Chelsea have not opened talks with the midfielder on a new deal.

They will be particularly reluctant to lose him now they have been banned by Fifa from signing new players for the next two transfer windows, over their inducements to the French teenager Gaël Kakuta to sign for the club and break his contract with the French club Lens in 2007.

However, Ballack, who will be 34 by the time the ban is lifted, insists he will not make a decision on his future until after South Africa in 2010.

The German was reported in the Daily Mail saying: "I will decide after the 2010 World Cup if I will continue playing or if I will stop. I will continue as long as I enjoy it and still have ambitions in the sport."