The Fir Park club rejected bids, one of £2m, from Celtic and Barnsley this week for the 19-year-old.

Turnbull was last month named Scottish Football Writers' young player of the year after scoring 15 goals in his breakthrough season at Fir Park.

The deal would smash Motherwell's previous club record sale of £1.7m for Phil O'Donnell to Celtic in 1994.

Turnbull will now discuss personal terms with the Scottish champions.

Scotland Under-21 cap Turnbull was free to talk to clubs in January when his deal was down to the final six months, but instead opted to sign a new contract until summer 2021.

Motherwell said in a statement: "We have continued to stand by what we believe is a fair value for David. In light of the negotiating stance we have adopted, the money we stand to receive will be transformational for us as we continue to operate as a fan-owned club."