Workers in critical industries such as social care, schools and the armed forces have been abandoned by the Government, unions have said.
GMB, which has more than 600,000 members, criticised the Budget for ignoring key worker wages, 'poverty' sick pay and the industries desperate for support.
Warren Kenny, GMB acting general secretary, said: "Warm words don't pay the bills," as he accused the Chancellor of neglecting those on the front line.
Many public sector workers face a pay freeze this year despite calls for an increase a year on from the coronavirus crisis.
The move was announced last November, affecting 1.3million people including teachers, police officers, firefighters, the armed forces, and council staff.
"The Chancellor might get some likes on Instagram, but he won't be getting any love from public service workers who will feel like this Budget is a kick in the teeth after everything they've done for the country throughout this pandemic," Kenny said.
"Not a penny extra will go to the pockets of our key workers, it's a national scandal."
In his spring Budget on Wednesday, the Chancellor delivered a pay rise to the lowest earners from April.
Speaking at a post-Budget Downing Street press conference, Sunak defended the public sector freeze.
The Chancellor said that while there had been a "pause" in public-sector pay increases outside the NHS, the majority in the public sector would still see their pay rise next year.
He said it was private businesses and staff bearing the economic brunt of the pandemic.
Sunak said: "Given that and given the very obviously difficult fiscal situation that we face, I thought for those reasons, and also to try to protect those public-sector jobs, it was reasonable to take a more targeted approach to public-sector pay this year."
But Rehana Azam, national officer of the GMB union, said the Chancellor's Budget on Wednesday was an "insult" to public sector workers.
"This Budget is an insult to the millions of NHS, schools, care, local government workers who have seen us through this crisis."
Sunak was also accused of ignoring the millions of people who have not qualified for sick pay throughout the pandemic.
"The lack of action on the super spreader policy of poverty Statutory Sick Pay rates is simply an abrogation of duty," GMB said.
"You can't talk about helping the poorest and leave this unchanged. Those who can't afford to self-isolate and pay the bills won't do so, they will feel they have no choice but to risk going to work rather than get into debt.
"We keep being told we're not out of the woods yet - I agree - so it’s shocking that this Budget did nothing to address this."
