The ‘London Loo League Table’ report warns that less than a quarter of stations in zones 1-3 have toilets, and that their availability varies wildly between Tube lines.
Authored by Caroline Russell, a Green member of the London Assembly, the research says that while 83 per cent of Metropolitan line stations have loos, the same is true of just 27 per cent on the Northern line.
The longest stretch without toilets was found on the Northern line between Morden and Kennington, with passengers having to wait until the next stop at Elephant & Castle to find facilities.
Another ‘loo desert’ is found on the District line between Wimbledon Park and Bayswater - a gap of a 10 stops, taking 22 minutes.
Even the most modern stations can lack loos, the report points out - with the Elizabeth line missing toilets at three stations, including Bond Street and Canary Wharf.
Ms Russell said: “Londoners shouldn’t have to cross their legs and hope on the Tube.
“A properly accessible public transport system must include toilets and information on where to find them.
“This is essential for everyone, especially older and disabled people who may be restricted from travelling by a ‘loo leash’.”
She added: “Delay and dithering on the Mayor’s feasibility study is frustrating, Londoners need the Mayor to get on with making our tube network flush with new toilets.”
The study referred to by Ms Russell was promised by Sadiq Khan in February, after Labour Assembly Members voted down a Green proposal to immediately create 70 new toilets across the transport network, at a cost of £20m.
Len Duvall, the Assembly’s Labour group leader, had said the proposal “just seemed too vast” to commit to without first carrying out a feasibility study.
In May, TfL said it would share the terms of reference for the study by the end of June, but then said in August that it would be doing so “in the coming weeks”.
In response to Ms Russell's report, Mark Evers, TfL's chief customer officer, said: “We recognise that toilet provision is absolutely essential for customer care and particularly so for older and disabled customers.
"We are working at pace on our study into improving toilets, and are committed to sharing this with our stakeholders and to working closely with them to gain a greater understanding of where we can improve facilities and identify suitable further opportunities for more toilets.”
A spokeswoman for Mr Khan said the mayor “recognises the importance of free and accessible public toilets for Londoners” and that he will continue working with TfL to identify opportunities for more toilets.
The ‘Loo League Table’ of lines on the Tube map, given in the report, is as follows:
1.) Elizabeth Line 95 per cent (93 per cent accessible)
2.) Metropolitan - 83 per cent (29 per cent accessible)
3.) Jubilee - 63 per cent (52 per cent accessible)
4.) Central - 59 per cent (16 per cent accessible)
5.) Piccadilly - 55 per cent (19 per cent accessible)
6.) Waterloo & City - 50 per cent (50 per cent accessible)
7.) Victoria - 44 per cent (25 per cent accessible)
8.) District - 42 per cent (25 per cent accessible)
9.) Bakerloo - 32 per cent (24 per cent accessible)
10.) Circle - 29 per cent (23 per cent accessible)
11.) Overground - 28 per cent (26 per cent accessible)
12.) Hammersmith & City - 28 per cent (24 per cent accessible)
13.) Northern Line - 27 per cent (21 per cent accessible)
14.) DLR - 11 per cent (11 per cent accessible)
15.) Tram - 10 per cent (8 per cent accessible)