Only Ramsay MacDonald’s 1923 minority government had a narrower base of support.

1923 MacDonald 30.7%
2024 Starmer 33.7%
2005 Blair 35.2%
2010 Cameron 36.1%

If you look not at percentage of voters but percentage of the total electorate, the result is even more stark.

2024 Starmer 20.2%
2005 Blair 21.8%
1923 MacDonald 21.8%
2010 Cameron 26.7%

One in five eligible votes cast their votes for Starmer. If you look at the other lowest examples, MacDonald led a minority government, Cameron a coalition and Blair had to resign after two years.