England batsman Dawid Malan has stepped down as Middlesex captain.
Middlesex finished eighth in County Championship Division Two and Malan, 32, said missing out on promotion had been "extremely disappointing".
His decision to resign was announced at an end-of-season event where Middlesex revealed that West Indies seamer Miguel Cummins has signed a three-year deal.
Spinner Nathan Sowter has signed an extension until 2021 but bowlers Ollie Rayner and Tom Barber are to leave.
Malan took over the captaincy at Lord's in February 2018 and led Middlesex to a fourth-place finish that year, but they won just three County Championship games in 2019.
However, they reached the knockout stages of both the One-Day Cup and the T20 Blast this summer.
A statement from Malan read: "Middlesex played some fantastic cricket during the 2019 season, reaching the knockout stages of both white-ball competitions - achievements of which we can be proud. However, our form in the second division of the Championship was inconsistent.
"I have therefore decided to step down as captain and to allow someone else the opportunity to work with the coaching staff to readjust and hopefully guide the team back towards the top of the first division."
Malan, a one-county player who has been with Middlesex since 2006, has scored 724 runs in 15 Tests for England and made his one-day international debut against Ireland earlier this year.
In January, he signed a new Middlesex contract until 2021, which would extend his stay at the county to 15 years.
Angus Fraser, Middlesex's managing director of cricket, said: "Dawid has shown great pride and dignity in the way he has handled a difficult season and he leaves the role knowing he has given it all he can.
"As a batsman the captaincy seems to have got the best out of him and he has had an excellent season. He has been our best player in testing batting conditions."
Pace bowler Cummins, 29, joined Middlesex at the end of the 2019 season, taking eight wickets in their final three Championship games, and has signed a contract until 2022.
Cummins joins as a non-overseas player under the Kolpak ruling - meaning he will have to give up his West Indies career even though he played in their most recent Test series last month.
Australia-born leg-spinner Sowter has taken 97 wickets in 80 appearances across all formats for Middlesex since 2015 and the 26-year-old will remain at the club for at least two more years.
However, long-serving off-spinner Rayner, 33, who has been with the club for eight years, and fast left-armer Barber, 24, have been informed that their contracts will not be renewed.