Brendon McCullum is to have surgery on his back, forcing him out of the remainder of Middlesex's T20 Blast campaign.

The former New Zealand captain was troubled by the injury during a short spell with Middlesex earlier this season, and the problem has worsened while he was playing in the Caribbean Premier League.

The big-hitting 34-year-old will miss the Blast quarter-final against Northamptonshire on Wednesday, when a place in Finals Day at Edgbaston later this month will be at stake.

Middlesex managing director of cricket Angus Fraser says he was aware McCullum was struggling with his back.

"The short nature of a T20 means there is always the temptation to play someone when they have an injury, but Brendon felt he could not do himself justice and did not want to risk letting the team down in such an important game.

"He has been having regular injections and dosing up on pain killers for some time just to get by, which is not particularly good for you."