Mr Walker's move comes as Prime Minister Boris Johnson struggles to maintain a grip on power in No 10 as ministers and aides continue to quit his Government in protest at his leadership.
Treasury minister John Glen later resigned, telling Mr Johnson “I can no longer reconcile my commitment to the role” with “the complete lack of confidence I have in your continuing leadership of our country”.
Will Quince has resigned as children and families minister, saying he could not accept being sent out to defend the Prime Minister on television with inaccurate information over the Chris Pincher row.
Laura Trott has quit as a ministerial aide, saying “trust in politics is – and must always be – of the utmost importance, but sadly in recent months this has been lost”.
Their resignations on Wednesday morning followed a string of departures from the Government on Tuesday evening, led by ex-Chancellor Rishi Sunak and former Health Secretary Sajid Javid who delivered broadsides at Mr Johnson as they quit their Cabinet posts.
Robin Walker's resignation comes as another blow to the PM PA
Sajid Javid and Rishi Sunak both resigned on Tuesday Toby Melville
Mr Quince was one of the ministers sent on the airwaves to defend Mr Johnson’s position over Chris Pincher, who quit as deputy chief whip after allegedly assaulting two men while drunk at London’s Carlton Club.
The Prime Minister later acknowledged he had previously been informed of allegations against Mr Pincher dating back to 2019 and said he regretted keeping him in government beyond that point.
Mr Quince said he had received a “sincere apology” from Mr Johnson for being sent out with an “inaccurate” briefing about the Prime Minister’s knowledge of events.
But “I have no choice but to tender my resignation” as “I accepted and repeated those assurances in good faith”.
The Prime Minister’s authority had already been damaged by a confidence vote which saw 41% of his own MPs withdraw their support in June.
The loss of crunch by-elections in Tiverton and Honiton and Wakefield later that month triggered the resignation of party chairman Oliver Dowden, while there is still lingering anger over coronavirus lockdown-busting parties in Downing Street.
Tory MPs are also uneasy about the Government’s high spending, high taxing approach as a result of the response to the pandemic.