13.—(1) Where a contracting authority intends to award a contract on the basis of the offer which is the most economically advantageous in accordance with regulation 31(1)(a), it shall indicate in the contract notice whether or not it authorises economic operators to submit offers which contain variants on the requirements specified in the contract documents and a contracting authority shall not accept an offer which contains a variant without that indication.
(2) Where a contracting authority authorises a variant in accordance with paragraph (1) it shall state in the contract documents the minimum requirements to be met by the variants and any specific requirements for the presentation of an offer which contains variants.
(3) A contracting authority shall only consider variants which meet its minimum requirements as stated in the contract documents in accordance with paragraph (2).
(4) A contracting authority shall not reject an offer which contains variants on the requirements specified in the contract documents on the ground that—
(a)where it intends to award a services contract, the offer would lead to the award of a supply contract; or
(b)where it intends to award a supply contract, the offer would lead to the award of a services contract.