Arsenal are reportedly locked in negotiations with Celtic over the transfer of Kieran Tierney after the Glasgow club rejected a £25m bid for the left-back.

Tierney, who became a regular for Celtic in 2015, is viewed by Unai Emery as a key target as the Spaniard seeks to add greater muscle to his defence.

The Gunners have reportedly already made two separate offers which were both rejected, with the first one being £15m and the second closer to £18m.

According to the Evening Standard, Arsenal remain locked in negotiations with Celtic for the player after a third bid worth £25m was turned down this week.

That figure meets Celtic's valuation of the player, but the Scottish champions want more money upfront as opposed to the series of add-on payments being proposed by Arsenal.

Arsenal's transfer window has been complicated by having a nominal budget of just £40m, which has forced the club to negotiate deals on the basis of installments.