The focal point for Welsh social and cultural life in London for more than 70 years could close due to falling support, officials have warned.
It is thought the London Welsh Centre, in the London borough of Camden, could shut in 18 months if trade continues to drop.
Organisers are now looking to reverse its decline.
The London Welsh Centre, in Gray's Inn Road, was set up to serve the Welsh community living in and around Camden.
The area was historically the hub of the Welsh dairy industry in London.
During World War II, the building acted as a base for Welsh soldiers passing through the city.
It now provides a meeting place for people of Welsh descent living in the city and hosts Welsh language classes and social events.
It is also home for a number of choirs and cultural groups.
In recent years, however, membership has fallen and the centre has suffered from a funding shortfall.
An ultimatum has now been issued to the centre's members, urging them to "use it or lose it."
"The situation is critical," said London Welsh Centre chairman Barrie Lloyd.
"Put very bluntly, if we take no steps to improve the situation the centre will have to close its doors
A crisis committee has been set up and will meet on a monthly basis to see if the centre can be saved.
An extraordinary general meeting will also be held in the summer to decide whether it has a viable future or should be wound up.
"We're caught between two stools," said Mr Lloyd.
"In the past we've had problems securing funding from charitable organisations in Wales because we're based in London - but English organisations regard us as Welsh."
Officials say money needs to be spent on modernisation if the centre is to attract more visitors.
"Maintenance costs money and the centre is starting to look dated," said Mr Lloyd.
"Ironically, I've been told that some Welsh organisations in London are choosing to meet elsewhere because they think the centre is too shabby."
Supporters say the closure of the centre would be devastating for the Welsh community in London.
"I'm not prepared to imagine life for the London Welsh without the centre," said BBC News presenter Huw Edwards.
"It would be an enormous blow if the centre closed and people would really feel its absence.
"It's been the heart of Welsh life in the city for decades. I refuse to think about closure and I hope that by setting out the reality of situation people will wake up and start doing their bit to keep the centre open."
Members are being urged to come forward with ideas on how to save the centre and to donate time or money.
These will be incorporated into a new business plan aimed at reviving the centre's fortunes.