I also recorded from 2015 onward some media reports, but now have adopted the policy of only using HOCL and accounts numbers, as this other data cannot be assumed to be reliable, eg inconsistent handling of members in arrears, or originating from unreliable leaks possibly to make some factional point. I have left this now unused data in below, but commented out.
Note the Labour Party Rule Book states that membership lapses after arrears of six months to give time for members to be reminded to renew, so there will be a significant difference between currently fully paid-up member numbers (part of eligibility to vote in internal elections) and the total membership number. Also there will be some delay from a member deciding not to renew and removal from membership data.
Note that I think there was a new basis for producing the numbers in 1980, from a summation of membership data recorded at the constituency level, to a more accurate centralised individual membership database.
# Date, LabourMembers, Other data source 1928-12-31, 215000 1929-12-31, 228000 1930-12-31, 277000 1931-12-31, 297000 1932-12-31, 372000 1933-12-31, 366000 1934-12-31, 381000 1935-12-31, 419000 1936-12-31, 431000 1937-12-31, 447000 1938-12-31, 429000 1939-12-31, 409000 1940-12-31, 304000 1941-12-31, 227000 1942-12-31, 219000 1943-12-31, 236000 1944-12-31, 266000 1945-12-31, 487000 1946-12-31, 645000 1947-12-31, 608000 1948-12-31, 629000 1949-12-31, 730000 1950-12-31, 908000 1951-12-31, 876000 1952-12-31, 1015000 1953-12-31, 1005000 1954-12-31, 934000 1955-12-31, 843000 1956-12-31, 845000 1957-12-31, 913000 1958-12-31, 889000 1959-12-31, 845000 1960-12-31, 790000 1961-12-31, 751000 1962-12-31, 767000 1963-12-31, 830000 1964-12-31, 830000 1965-12-31, 817000 1966-12-31, 776000 1967-12-31, 734000 1968-12-31, 701000 1969-12-31, 681000 1970-12-31, 680000 1971-12-31, 700000 1972-12-31, 703000 1973-12-31, 665000 1974-12-31, 692000 1975-12-31, 675000 1976-12-31, 659000 1977-12-31, 660000 1978-12-31, 676000 1979-12-31, 666000 1980-12-31, 348000 1981-12-31, 277000 1982-12-31, 274000 1983-12-31, 295000 1984-12-31, 323000 1985-12-31, 313000 1986-12-31, 297000 1987-12-31, 289000 1988-12-31, 266000 1989-12-31, 294000 1990-12-31, 311000 1991-12-31, 261000 1992-12-31, 280000 1993-12-31, 266000 1994-12-31, 305000 1995-12-31, 365000 1996-12-31, 400000 1997-12-31, 405000 1998-12-31, 388000 1999-12-31, 361000 2000-12-31, 311000 2001-12-31, 272000 2002-12-31, 248000 2003-12-31, 215000 2004-12-31, 201000 2005-12-31, 198000 2006-12-31, 182000 2007-12-31, 177000 2008-12-31, 166000 2009-12-31, 156000 2010-12-31, 193000 2011-12-31, 193000 2012-12-31, 187000 2013-12-31, 189531, 2014 accounts: http://search.electoralcommission.org.uk/Api/Accounts/Documents/16479 2014-12-31, 193754, 2014 accounts: http://search.electoralcommission.org.uk/Api/Accounts/Documents/16479 #2015-05-06, 201293, https://www.theguardian.com/politics/2016/jan/13/revealed-how-jeremy-corbyn-has-reshaped-the-labour-party #2015-08-25, 292505, http://www.mirror.co.uk/news/uk-news/four-labour-leadership-candidates-rule-6316659 2015-12-31, 388262, 2015 accounts: http://search.electoralcommission.org.uk/Api/Accounts/Documents/17488 #2016-01-10, 388407, http://www.theguardian.com/politics/2016/jan/13/revealed-how-jeremy-corbyn-has-reshaped-the-labour-party #2016-06-30, 450000, http://www.independent.co.uk/news/uk/politics/labour-party-gains-60000-new-members-following-attempted-coup-against-corbyn-a7112336.html #2016-07-07, 503143, http://www.theguardian.com/politics/2016/jul/08/jeremy-corbyn-stands-defiant-after-labour-membership-surge 2016-12-31, 543645, 2016 accounts: http://search.electoralcommission.org.uk/Api/Accounts/Documents/19405 #2017-03-01, 528180, https://www.theguardian.com/politics/2017/mar/03/labour-party-lost-members-mid-2016 #2017-03-26, 517000, http://researchbriefings.parliament.uk/ResearchBriefing/Summary/SN05125 (2017-03-28) #2017-06-13, 552000, http://www.huffingtonpost.co.uk/entry/labour-party-membership-soars-by-33000-in-four-days-since-general-election_uk_59400feee4b0e84514ee930f #2017-07-18, 575000, Labour NEC member: https://aliceperry.wordpress.com/2017/07/20/labour-nec-report-july-2017/ #2017-09-20, 569000, https://www.totalpolitics.com/articles/opinion/kevin-maguire-peace-could-yet-break-out-labour-conference #2017-11-26, 568500, Labour NEC member: http://www.leftfutures.org/2017/12/pete-willsman-reports-from-labours-november-executive/ 2017-12-31, 564443, 2017 accounts: http://search.electoralcommission.org.uk/Api/Accounts/Documents/20546 #2018-04, 540000, HoC research report repeating Labourlist estimate: https://researchbriefings.parliament.uk/ResearchBriefing/Summary/SN05125 #2018-09-03, 540000, HoC research report repeating Labourlist estimate: https://commonslibrary.parliament.uk/parliament-and-elections/parliament/uk-political-party-membership-figures-august-2018/ #2018-12-31, 518000, Labour NEC member: https://labourlist.org/2019/07/selections-membership-figures-antisemitism-alice-perrys-latest-nec-report/ 2018-12-31, 518659, 2018 accounts: http://search.electoralcommission.org.uk/Api/Accounts/Documents/21510 #undated - published 2019-07-23, 485000, https://www.politicshome.com/news/uk/political-parties/labour-party/news/105507/excl-labour-membership-dips-below-half-million 2019-12-31, 532046, 2019 accounts: http://search.electoralcommission.org.uk/Api/Accounts/Documents/22602 #2020-01, 580000, Labour NEC member: https://labourlist.org/2020/01/what-happened-at-labours-nec-post-election-away-day/ #2020-06-30, 580000, Labour NEC member: https://labourlist.org/2020/06/action-over-leaked-report-virtual-meetings-nec-elections-alice-perrys-nec-report/ #2020-07, 430000, https://web.archive.org/web/20210801070808/https://www.thetimes.co.uk/article/huge-legal-bills-leave-labour-feeling-the-pinch-vfx2qh97h #2020-08-24, 495961, Eligible voters in NEC election published 2020-11-12: https://labour.org.uk/wp-content/uploads/2020/1/M1824_1_ReportofVoting_1311_NEC.pdf 2020-12-31, 523332, 2020 accounts: http://search.electoralcommission.org.uk/Api/Accounts/Documents/22905 #2021-01, 400000, https://labourlist.org/2022/01/labour-beset-by-difficulties-as-party-does-not-have-full-up-to-date-membership-list/ 2021-12-31, 432213, 2021 accounts: http://search.electoralcommission.org.uk/Api/Accounts/Documents/24331 #2022-07, 415000, Andrew Fisher: https://twitter.com/FisherAndrew79/status/1550137089689370624 2022-12-31, 407445, 2022 accounts: https://search.electoralcommission.org.uk/Api/Accounts/Documents/25317 #2023-05-23, 395811, GenSec report to NEC (including in arrears): https://labourlist.org/2023/06/labour-party-members-numbers-membership-how-many-big/ #2023-07, 399195, GenSec report to NEC (including in arrears): https://labourlist.org/2023/07/labour-nec-report-forde-npf-by-elections-conference-keir-starmer/
This graph was produced using gnuplot, running on Linux. The gnuplot input/program is given below. I am a gnuplot novice and any improvements offered would be welcome.