Off-spinner Shaun Udal has stepped down as Middlesex captain with immediate effect.
Udal, 41, has been in charge since September 2008 but has come to a mutual agreement to relinquish the job following a disappointing start to the season.
Veteran Australian wicketkeeper Adam Gilchrist - currently at Lord's on a short-term contract - is to take over for the next six matches, five in the FP t20 and a 50-over tour game against his countrymen on June 19.
Neil Dexter will then assume command for the rest of the season, his first game in charge being the FP t20 fixture away at Glamorgan on June 26.
"Shaun took over the captaincy at an extremely difficult time for the club and has led the side with energy and pride," said Middlesex managing director of cricket Angus Fraser.
"During the two years he has been in charge he has helped create a united team and a positive environment.
"He now has a key role to play in assisting his successor, Neil Dexter.
"Shaun cares passionately about how Middlesex CCC performs and has worked extremely hard to turn the team in to a winning outfit but the indifferent cricket we continue to play has worn him down.
"It has affected his bowling too. Shaun is an intelligent and articulate man and he feels the team needs a change in leadership.
"His decision is respected by everyone at Middlesex CCC who would all like to thank him for the commitment he has shown during his time in charge.
"Shaun is not having the best of seasons but he still has a role to play on the field. Unless I hear otherwise he is available for selection for teams that will be picked on merit.
"Neil Dexter was identified as a worthy successor to Udal earlier in the season and he will captain the club once Adam Gilchrist leaves.
"Neil is an extremely impressive young man. He is a fine player and is highly respected by everyone at the club."
On Gilchrist, Fraser added: "It would be foolish for Middlesex CCC not to make use of one of the most experienced and respected cricketers the game has produced whilst he is at the club.
"He captained the Deccan Chargers to success in the 2009 Indian Premier League and led Australia in all forms of the game with distinction on numerous occasions. In his short time in England he has already had an influential role in the t20 cricket we are playing.
"Everything about Adam is 'world class' and since his arrival he has had an extremely positive effect on the dressing room. Our players love having him around and playing in the same side as Adam shows our young cricketers what is required to be a great player.
"Adam's presence gives Neil a wonderful opportunity to watch and learn from one of the finest and most respected players to play the game."
Udal, who remains available for selection, reflected with pride on his spell in charge, which saw him stabilise the side following a difficult period under predecessor Ed Smith.
"Captaining Middlesex CCC has been a huge honour for me," said Udal.
"There can be few better feelings than leading a team out at Lord's and I have taken great pride in doing that over the course of the past three seasons.
"Letting go of something like that is extremely difficult but Angus Fraser and I have agreed that this is the best way forward for the club.
"I would like to continue playing for Middlesex and will give my full support to both Adam Gilchrist and Neil Dexter."