TNS had resisted the meeting and previous offers by WPP in favour of pursuing the merger it has agreed with Germany's market research giant GfK.
The meeting must be held to comply with an order from the Takeover Panel that TNS has to disclose to WPP the same information it has provided to GfK's largest shareholder, GfK Verein.
This means WPP, which owns market research group Kantar, will be given detailed information about TNS's business in a presentation by management.
This will help Sorrell decide whether to make a new bid for TNS in an attempt to derail its proposed merger with GfK, or to walk away.
TNS rejected a £996m cash and WPP shares bid from Sorrell in the middle of May. The fall in WPP's share price since then from 638.5p to 559.5p this morning means a new bid will be more expensive for the group.
Last week, Sorrell said that he was still weighing up his options. He has criticised TNS and GfK for pursuing the merger and questioned their ability to deliver the £76m cost savings they say are possible.
A spokesman for TNS said last night: "TNS and its advisers will act in the interests of its shareholders at all times and that is what they will be doing tomorrow."
Last week TNS founder Liz Nelson criticised TNS and backed Sorrell's bid for the British market research group.