Her former North West Durham constituency Labour Party passed a motion to expel her at a meeting on Friday – when her successor, Laura Pidcock, remained silent, according to witnesses.
Mr Watson, Labour's deputy leader, made his fury known by tweeting: “This misdirected mob justice degrades our great party.
“These attempts to silence dissent will fail. This is not the party I know.”
Fellow Labour MPs also criticised the move. Anna Turley, a North East MP, tweeted: “Hilary has served the Labour Party with commitment & distinction for decades, as a former social worker, particularly for the vulnerable.
“She is Labour to her marrow and if there is no place for her in it, it is done.”
And Pat Glass, who also represented North West Durham for Labour, tweeted: “I cannot believe that @NWDurhamLabour have passed a motion to expel former MP @hilaryarmstrong but it seems it is true.”
“She has broken no rules unless free speech is now banned in the @UKLabour”.
Now ennobled as Baroness Armstrong, the former chief whip joined the revolt in the House of Lords which has deepened the antisemitism crisis dogging the Labour leader.
Mr Corbyn is now facing a potential vote of no confidence by Labour peers this week, after Baroness Hayter, the shadow Brexit minister, was sacked for likening his leadership to the “last days of Hitler”.
In a statement, Baroness Armstrong said: “I'm very sad to hear this. North West Durham constituency Labour Party always used to be a place where differing opinions were respected.
“This sadly shows how far the party has departed from its roots. I'm not going to shy away from demanding that the party leadership take real and decisive action to rid antisemitism from the Labour Party.”
A Labour source said the constituency Labour Party does not have the power to expel, so the motion had no formal standing.