Overcrowding on London Underground will get worse because of plans to pause upgrade work, it has been warned.
The drivers' union Aslef said putting on hold new trains for the Northern and Jubilee lines made "outright cancellation" more likely.
LU said its plans were being temporarily paused.
Finn Brennan, Aslef's organiser on LU, said: "It is deeply disappointing and looks like a classic example of making medium-term savings that damage the long-term ability of the Tube to cope with London's growth.
"This upgrade was to involve the purchase of 17 additional trains so the Northern line could run a 30-trains-per hour service on all branches of the line.
"The Jubilee line would have had 10 new trains to operate a 36-per-hour service."
"The Northern line is one of the most overcrowded parts of the Underground and passenger demand is projected to grow by a further 25% by 2023.
"Without new trains, overcrowding will get worse and worse and the Northern line risks returning to the days when it was known to millions of Londoners as the "misery line".
"The removal of London Underground's operating subsidy by then chancellor George Osborne and the continued government squeeze on Transport for London's budget is now having a devastating impact on future investment."
David Hughes, LU's director for strategy and network development, said: "The benefits from the largest investment programme in the Tube's history, combined with the huge capacity increases coming soon from the arrival of the Elizabeth line, means that we are temporarily pausing our plans to buy more trains for the Jubilee and Northern lines.
"Our modernisation programme and ?in-depth technical work by our new combined engineering division has improved performance and reliability.
"This means we can increase the frequency of our existing trains without the immediate need for new trains, making journeys quicker and more comfortable for thousands of customers every day."