Britain called on Chad on Saturday to arrest Sudanese President Omar al-Bashir while he visits N'Djamena for a regional summit and to hand him over to the International Criminal Court.
Chad is one of the countries signed up to the ICC's founding treaty which are legally bound to arrest Bashir, who is wanted for war crimes, crimes against humanity and genocide in the long-running Darfur conflict.
But several signatories have failed to do so, and Bashir visited Chad on Saturday for talks with central African leaders on security, including the operation against Islamists in Mali.
"If President Bashir is not arrested, this will be the third time the government of Chad has failed to implement warrants issued by the International Criminal Court for war crimes, crimes against humanity and genocide," British minister for Africa Mark Simmonds said.
"Chad has committed itself to full cooperation with the ICC, and I reiterate the importance that the British government places on such commitments.
"The UK expects Chad to stand by its obligations and will be disappointed if it does not do so."
The ICC on Friday issued a formal notice to the Chadian government reminding it of its obligation to arrest Bashir.
A note was also sent to Libya amid reports Bashir will also visit there this weekend, although Libya is not a signatory to the ICC.