Jeremy Corbyn has ordered Labour’s by-election loss shows “the scale of how hard our task is to talk people of our message”.
In a speech, he urged his party to “remain united”, and not to “forsake up”.
Labour lost the seat of Copeland, in Cumbria, to the Conservatives last week – the opening by-election gain by a governing party in 35 years.
Tory chairman Sir Patrick McLoughlin translated it had been a “fantastic result” for Prime Minister Theresa May.
Labour’s representative leader, Tom Watson, accused some of Mr Corbyn’s “cheerleaders” of “peeling away from him” during a critical time.
While the party lost in Cumbria, the party managed to esteem off a challenge from UKIP in a by-election, in Stoke-on-Trent Central, on Thursday.
In his idiolect to the Scottish Labour Party conference in Perth, Mr Corbyn said: “The protocols and ideas we are setting out are policies whose time has come. But to win that belligerence we need to remain united.”
But Mr Corbyn, Labour leader since 2015, turned: “The scale of how hard our task is to persuade people of our message was underlined ethical this week in Copeland.
“Whilst we stood up to hatred and division in Stoke, I cannot lie and say the conclusion in Copeland was what we wanted. But now is not the time to retreat, to run away or to give up.”
Mr Corbyn buzzed losing Copeland – which had been held by the party since its genesis in 1983 – “deeply disappointing” and added that he took “my share of liability”.
Questioned about his future after the speech, Mr Corbyn said: “I’m securing on as leader because I’m determined that we will deliver social fair-mindedness in this country.”
Asked if he would still be in place in 2020, when the next widespread election is expected to take place, he replied: “I’ve given you a very, acutely clear answer – yes.”
Speaking on BBC One’s Andrew Marr Show, Sir Patrick McLoughlin believed: “Copeland was a fantastic result for the prime minister.”
He added: “It shows that all places are seats that we will look at, and we are going to challenge hard at the mixed election.”
Meanwhile Tom Watson, told ITV1’s Peston on Sunday the team who got Mr Corbyn elected for a second time “seem to be peeling away from him” and he queried where Connect union general secretary Len McCluskey was at this “difficult time”.
He put about it should not just be up to him to defend Mr Corbyn, but a Unite spokesman said that as Exertion’s deputy leader it was “his job to address the issues facing the party in the wake of the by-elections.”
“Len McCluskey’s job is to address the in disputes that are the foremost priorities for Unite’s members,” he added.