Labour under Jeremy Corbyn's leadership is further from power than any time in the last 50 years, says former South Shields MP David Miliband.
As Mr Corbyn made clear he is determined to continue as Labour leader despite a humiliating defeat in the Copeland by-election, Mr Miliband said he was "deeply concerned" about the party.
The currently US-based former foreign secretary also refused to rule out a future run for the Labour leadership although admitted a return to frontline politics was "hard to see" at present.
Mr Miliband told The Times: "I'm obviously deeply concerned that Labour is further from power than at any stage in my lifetime."
Asked about his future, he added: "I honestly don't know what I'm going to do.
"It's hard to see, but what's the point of saying never?"
MPs have warned the party was on course for a "catastrophic" general election defeat after the Conservatives snatched the Cumbrian seat of Copeland which had been held by Labour since 1935.
Mr Corbyn said the result was "very disappointing" but made clear he would resist calls to resign.
"I was elected to lead this party. I am proud to lead this party," he told reporters.
"We will continue our campaigning work on the NHS, on social care, on housing."
One leading trade union figure warned Mr Corbyn he had to "turn things around" and show he could deliver a Labour government, while another said he needs to take responsibility for improving the party's fortunes "sooner rather than later".