David Miliband has refused to rule out a comeback to British politics as he said Labour was languishing further from power than at any point in the last 50 years.

The former foreign secretary, who now runs a charity in New York, said he was “deeply concerned” about the state of the party.

Miliband, who stepped down as an MP in 2013 after narrowly losing the leadership election to his brother, Ed, also told The Times a return to the Commons was difficult to imagine but he would not “say never”.

He was speaking in the aftermath of two crucial by-elections, in which Labour saw off the challenge of a chaotic UKIP party in Stoke-on-Trent Central but relinquished the Copeland seat it has held for more than 80 years. Theresa May travelled to the west Cumbrian constituency today to celebrate with new MP Trudy Harrison.

Miliband – who won the first round of the leadership vote in 2010 before losing in the run-off – has now issued a gloomy verdict on his party’s prospects.

“I’m obviously deeply concerned that Labour is further from power than at any stage in my lifetime”, he said.

When asked about his future, the chief executive of the International Rescue Committee charity said: “I honestly don’t know what I’m going to do. It’s hard to see, but what’s the point of saying never?”

Corbyn today vowed to continue in post as allies took to television and radio to confirm he would stay on.

John McDonnell said the party leader had never offered to resign while Corbyn said: “I was elected leader of this party, I’m proud to lead this party.”

The Tories achieved a 6.7 per cent swing in Copeland prompting a wave of criticism from Labour backbenchers. Corbyn’s allies insisted his policies could help turn around the party’s poll rating.