Overture Services has won an injunction from a German court forcing T-Online to reinstate the Overture search service on its home page.
T-Online replaced Overture's search listings with Google on 7 August, after it was announced that Yahoo was buying Overture.
A clause in the contract allowed T-Online to break the agreement when ownership of Overture changed, T-Online said, adding that it viewed Yahoo as a direct competitor.
Overture, however, insisted that T-Online is unable to break its contract.
"We've had the contract since August 2001 and had a good relationship. We were in talks to expand the deal and then, when the Yahoo merger was announced, they said they would use the change of control clause to cancel. We were totally surprised by that action," Overture Germany managing director Manfred Klaus said.
"They [T-Online] see Yahoo as a competitor but we don't agree. The deal with Yahoo hasn't even closed yet. This was so surprising for us, and we tried to talk to them and clarify it; the contract is clear and straightforward," Klaus added.
After talks failed, Overture applied for, and was granted, an injunction against T-Online.
There is no deadline for T-Online to comply with the judgment, but it must do so "as fast as possible, within the next few days", Klaus said.
"From there we will be happy to talk and settle things. We had a good relationship and we're still very interested in the agreement."
T-Online said, "We have not received any injunction so far. Once an injunction reaches us, it will be given to our legal department for thorough examination."
Yahoo's acquisition of Overture is due to close by the fourth quarter of 2003.
Gillian Law writes for IDG News Service