Shadow Brexit minister Jenny Chapman said Mr Johnson is the 18th minister to resign from Theresa May's government, saying: "She has lost all authority and is incapable of negotiating a Brexit deal within her own party, let alone with the EU".
"Of course I talk to colleagues across Parliament and those conversations always remind me of how deeply MPs think about their responsibilities, and I know many are reflecting hard about the deal that's looming and how they will respond to it".
His former foreign secretary brother may have been on the opposite side of the Brexit divide, but Boris Johnson has voiced his "boundless admiration" for him.
Prime Minister Theresa May this week faced pressure from her Westminster allies in the Democratic Unionist Party not to allow a customs border to split Northern Ireland from the United Kingdom after Brexit.
Robin Walker told the Funke group of newspapers that Brexit negotiations were progressing in a constructive manner and that there was agreement on more than 95 percent of the deal.
The remain-supporting brother of Boris Johnson, whose bombshell resignation took Westminster by surprise on Friday afternoon, said if other senior Conservatives followed suit, "good on them".
Tory former deputy prime minister Lord Heseltine, who backs the People's Vote campaign for another referendum, said: "This is a very significant resignation".
Many politicians are unhappy with her compromise plans to maintain the free trade of goods with the European Union, which they say will leave Britain subject to decisions in Brussels without any input.
Speaking to Matt Frei, Stanley Johnson reiterated that Jo resigned because the government is not delivering Brexit and because he can not support the Brexit plan "in honesty".
He accused the PM of offering a choice between "vassalage and chaos" and called for a second referendum now that the reality of Brexit had become clearer.
Walker said the European Union and Britain both needed to show flexibility and imagination. "My priority is really just to do my best as a now backbench MP to try and encourage the country to pause and reflect before we do something that is irrevocably stupid", Johnson said.