Museum bosses in Liverpool have pledged to fight proposed budget cuts which could force them to close two venues.
National Museums Liverpool (NML), which gets the majority of its funding from the government, has been told to expect cuts of between 30% and 40%.
It has now urged visitors to sign a petition against the cuts, due to be finalised at the end of October.
The Department for Culture, Media and Sport (DCMS) said culture would not be singled out in spending cuts.
NML has warned that Sudley House in Aigburth, the former home of Liverpool ship owner George Holt, could also move to opening only between April and October.
Officials said they accepted the organisation needed to work with less money, but have questioned the scale of the cuts being proposed.
"A 40% cut to us is absolutely massive," said spokesman Dickie Felton.
"It could potentially mean that we could close some of our venues. It's a very worrying time for us."
Hundreds of visitors have already signed the petition, started this week, and NML officials said they hoped a groundswell of support could sway the government's decision.
In a statement, a DCMS spokesperson said: "The government remains committed to maintaining free admission to the national museums and galleries.
"Putting the economy back on its feet and restoring the nation's finances are a priority for government, but minister are clear that arts and culture will not be singled out as soft targets."