Labour’s support is not currently high enough to win a general election, a spokesman for Jeremy Corbyn has said, but he insisted the party would see a change of fortune after the triggering of article 50.

His remarks echo the words of the shadow Brexit secretary, Keir Starmer, who said on Tuesday he believed the party had “no prospect” of winning the 2020 general election unless it changed its approach.

Speaking in the Commons shortly after the new Conservative MP for Copeland, Trudy Harrison, was formally sworn in, Corbyn’s spokesman was asked if he agreed with Starmer’s analysis.

“He said if nothing changes, then Labour won’t be winning a general election and clearly if the current level of support in the opinion polls were to continue that would clearly be the case,” the spokesman said.

In a YouGov poll published on Wednesday, the Conservatives lead Labour by 17 points, which would give the Tories a lead of more than a 100 seats on current boundaries.

Corbyn’s spokesman said he was confident the gap would close after the triggering of article 50 to start the process of leaving the EU at the end of March, but said the poor poll numbers were still a hangover from the leadership challenge to Corbyn over the summer.

“Obviously in the context of what took place over the summer and autumn ... then support fell back, and since Theresa May became prime minister, the Tories have opened up a significant lead, and in the byelections you can see the Ukip votes migrating to the Tory party,” he said.

However, he said that divisions in the Conservative party and the effect of a hard Brexit on the economy and wages would “come home to roost for the Tory party” and mean voters were more receptive to Labour’s message.

Other factors were also in play in the Stoke and Copeland byelections, the spokesman said, including the former Labour government and MPs who had held the seats. “The key ones relate to people’s experience of being neglected by successive governments, of the squeeze on living standards, in both Copeland and Stoke, and how there hasn’t been investment in those areas and the loss of trust that has built up over a significant period of time. In Copeland Labour support has been declining continuously since 1997,” he said.

Several MPs expressed anger at Monday night’s meeting of the parliamentary Labour party that previous Labour governments and MPs were being blamed for the defeat in Copeland, including by the shadow attorney general, Shami Chakrabarti, in her appearance on BBC1’s The Andrew Marr Show over the weekend.

Two MPs, Roberta Blackman-Woods and Pat McFadden, said they wanted to see the party put forward people who would defend the record of previous Labour governments, rather than blame them for the party’s misfortunes.

On Tuesday, Starmer told journalists he was not convinced by the reasons given by some in the shadow cabinet for losing the Copeland byelection, which included bad weather, a lack of car access for Labour voters and media appearances by Tony Blair and Peter Mandelson.

“I think the loss in Copeland was really serious. I don’t think some of the reasons put forward are compelling and I am very straightforward that if things don’t improve there is no prospect of us winning a general election,” he said.

“It is a very bad result for the Labour party and we need to be honest about that. A number of things came up, including the direction of travel of the Labour party, Labour’s ability to communicate and understand what people are saying to them and of course the leadership of the Labour party, and we all know that.”