Baroness Altmann said she was ‘horrified’ after reading a Treasury guide designed to educate people about saving.
Her dismay was caused, she said, by the absence of the word pension in the document.
She claimed UK private pensions are “in mortal danger”, and their “huge benefits seem under threat”.
She said: “Anyone who cares about pensions should be very, very worried.
“The Treasury has just released an infographic for the public, which shows how to save throughout the lifecycle, but doesn't mention the word 'pension'.
“This is further evidence of the concerns I have expressed before about Treasury attitudes to pensions. It suggests that our private pension system is under existential threat.
“During my time as Pensions Minister, there was clearly a difference of view between Treasury and DWP about private pensions.
“The Treasury sees them as a cost to the Exchequer. DWP sees them as a benefit for people to give them a better later life standard of living. That is how most people see them and why they are so important.”
She called for a major advertising campaign “to tell people why pensions are so valuable”.
She said: “The big problem with pensions is that many people do not appreciate their huge benefits.
“If you care about private pensions and believe they are worth fighting for, now is the time to stand up and shout about their benefits: Before it's too late and they are supplanted by an inferior product because of short-sighted policymaking that will leave long-term dangers.”
The Treasury said in a statement: "We back savers at every stage of their lives and it is our job to raise awareness of the savings products the public can access in the year ahead.
"It goes without saying that the Government supports pensions – that’s why we introduced auto enrolment, which is already benefiting more than seven million people, and our landmark pension freedoms."