The government has announced £30m to keep trams and metro trains running for key workers for 12 more weeks.
Millions of pounds in revenue have been lost due to the coronavirus lockdown. But Greater Manchester mayor Andy Burnham said the money was not enough.
The funding is split between the Tyne and Wear Metro, Manchester Metrolink, Sheffield Supertram, Midlands Metro and Nottingham Express Transit systems.
Transport Secretary Grant Shapps said protecting key transport was "vital".
The various light rail systems play "a crucial role in getting NHS staff and emergency services to work - and it is only right that we offer operators support during this time of national crisis," he said.
But Andy Burnham said he was "struggling to find words of support about this decision".
"At the start, we were told to spend what we needed to provide essential services, but now we find out we only have three-quarters of the income Metrolink needs to run at its current reduced services for key workers and essential journeys," he said.
"Once this funding ends in early June, we will simply not be able to continue running Metrolink.
"The public should be under no illusions that mothballing the largest light rail network in the country remains a very real possibility."
The total support package, backdated to mid-March, includes:
• Manchester Metrolink - £11.6m
• Tyne and Wear Metro - £8.6m
• Nottingham Express Transit - £3.7m
• Midland Metro - £2.1m
• Sheffield Supertram - £1.3m
Transport North East managing director Tobyn Hughes said keeping the Tyne and Wear Metro and Shields Ferry running for key workers had left a £10m "hole" in its budget.
"The government's funding of £8.6m goes a long way towards that and is very welcome, but it still leaves a funding gap that we need to close," he said.
"We do not however expect to return to pre-crisis levels of ridership for many months to come and this will continue to create a financial challenge long into the future."
Sheffield Supertram's managing director Tim Bilby said the service would play a key part in the region's recovery.
"It will be vital that the system has the resources it needs to seek to restore the service towards pre-pandemic levels when there will likely still be conditions which will change how people live and travel," he said.
Blackpool Transport says it would also ask for government support to protect the future of the resort's famous tramway, which has been suspended since 29 March, the Local Democracy Reporting Service said.
Blackpool North and Cleveleys MP Paul Maynard said the tramway was "far more than a tourist attraction" and could provide vital links for key workers.
Source: BBC