7 or more matches in a cricket series

Today (Sep 20) England begins a 7-match T-20I series against Pakistan. Is this the longest ever bilateral T20I series ?

Not quite. It is the first 7-match between ICC full members. However, there has been one earlier 7-match T20I series between Malawi and Mozambique in 2019, which was won 5-1 by Malawi with one no-result.

https://stats.espncricinfo.com/ci/engine/team/120.html?class=3;filter=advanced;orderby=start;spanmax1=10+Nov+2019;spanval1=span;template=results;type=team;view=results

What about ODI series ?

There has been one 8-match bilateral series between South Africa and Australia in 1993-94:

http://static.espncricinfo.com/db/ARCHIVE/1993-94/AUS_IN_RSA/

This was drawn 4-4. The 3-match Test series was also drawn 1-1

A “fake” 8-match ODI series was played in 1987-88 when the West Indies toured India in 1987-88 after the World Cup. This was actually a 7-match series but somehow the BCCI inserted an “extra” ODI at Ahmedabad between the 4th and 5th ODIs:

https://www.espncricinfo.com/series/west-indies-tour-of-india-1987-88-61942/india-vs-west-indies-only-odi-64317/full-scorecard

WI won this by 2 runs. It is considered an official ODI but not part of the series. The regular ODI series was won by the West Indies 6-1, although the 4-Test series was drawn 1-1 with the Hirwani and More show.

And Test series ?

There have been 21 6-Test bilateral series between 1970-71 and 1997-98. There was also the 9-Test triangular series between Eng, Aus and SA in 1912 but we are not counting that.

The 1970-71 Ashes was scheduled to be the first ever bilateral Test series with 6 Tests. This is what happened;

http://static.espncricinfo.com/db/ARCHIVE/1970S/1970-71/ENG_IN_AUS/

The 3rd Test at Melbourne was abandoned on the 3rd day due to rain. This led to the first-ever ODI being played a few days later, which was won by Australia.

https://www.espncricinfo.com/series/england-marylebone-cricket-club-tour-of-australia-1970-71-61725/australia-vs-england-only-odi-64148/full-scorecard

A “seventh” Test was tagged on after the scheduled 6th Test. However, most statisticians consider this to be a 6-Test series which was won by England 2-0, regaining the Ashes which they had lost in 1958-59.