II.23.2. Recovery procedure  II.23.3. Interest on late payment  II.23.4. Recovery rules in the case of joint tender  II.24. CHECKS AND AUDITS  43  The audit procedure is initiated on the date of receipt of the relevant letter sent  by the contracting authority. Audits are carried out on a confidential basis.  Such checks and audits may be initiated at any moment during the provision of  the services and up to five years starting from the payment of the balance of the  last specific contract issued under this FWC If the leader does not pay by the due date and if the amount cannot be offset in accordance  with Article II.23.2 (a), the contracting authority may claim the full amount to any other  member of the group by  notifying the debit note already sent to the leader under  Article II.23.2. If the contract is signed by a group (joint tender), the group is jointly and severally liable  under the conditions set out in Article II.6 (liability). The contracting authority first claims  the full amount to the leader of the group. Any partial payment is first entered against charges and interest on late payment and then  against the principal amount.  If the contractor does not honour the obligation to pay the amount due by the date set by  the contracting authority in the debit note, the amount due bears interest at the rate  indicated in Article II.21.8. Interest on late payments will cover the period starting on the  day after the due date for payment and ending on the date when the contracting authority  receives the full amount owed. If the contractor does not pay by the due date, the contracting authority may, after  informing the contractor in writing, recover the amounts due:  If no observations have been submitted or if, despite the observations submitted, the  contracting authority decides to pursue the recovery procedure, it must confirm recovery  by  formally notifying  a debit note to the contractor, specifying the date of payment. The  contractor must pay in accordance with the provisions specified in the debit note.  Before recovery, the contracting authority must  formally notify the contractor of its  intention to recover the amount it claims, specifying the amount due and the reasons for  recovery and inviting the contractor to make any observations within 30 days of receipt.  Contract number: ECHA/2020/666  (c) by taking legal action. (b) by calling in a financial guarantee if the contractor has submitted one to the  contracting authority; (a) by offsetting them against any amounts owed to the contractor by the Union or by  the European Atomic Energy Community; II.24.1  The contracting authority and the European Anti-Fraud Office may check or require  an audit on the  implementation of the FWC . This may be carried out either by  OLAF’s own staff or by any other outside body authorised to do so on its behalf.