Peter Mandelson, the architect of New Labour, has said he is working every day to undermine Jeremy Corbyn, as Blairite peers stood up in the House of Lords to lambast their party’s leadership for backing Brexit.

Mandelson, a former cabinet minister under Tony Blair and Gordon Brown, told an event for the Jewish Chronicle that he was actively working to bring an end to Corbyn’s leadership.

His comments will be inflammatory at a time when Labour MPs have toned down their public opposition to Corbyn, after his second leadership election by a huge margin in the autumn.

Mandelson, who has made no secret of his antipathy to Corbyn’s leadership, told the newspaper’s editor, Stephen Pollard: “The problem with Jeremy is not that he is a sort of maniac – it’s not as though he is a nasty person. It’s that he literally has no idea in the 21st century how to conduct himself as a leader of a party putting itself forward in a democratic election to become the government of our country.”

He added: “Why do you want to just walk away and pass the title deeds of this great party over to someone like Jeremy Corbyn? I don’t want to, I resent it, and I work every single day in some small way to bring forward the end of his tenure in office.

“Something, however small it may be – an email, a phone call or a meeting I convene – every day I try to do something to save the Labour party from his leadership.”

In response, a Labour source hit back at the peer, suggesting Mandelson was part of the establishment. “The idea of Jeremy Corbyn being prime minister and implementing policies that actually benefit the people terrifies the establishment, so it’s no surprise Peter Mandelson has found time in his busy schedule of spending time on oligarch’s yachts to attempt to undermine him,” the source said.

There is little appetite for another challenge to Corbyn among MPs, but a number of peers expressed fury about the leadership’s position on Brexit on the second day of debate on the Brexit bill in the House of Lords.

In defiance of the shadow cabinet’s collective support for triggering article 50, Roger Liddle, a former special adviser on Europe to Blair and now a peer, said Corbyn had failed to fight plans for a hard Brexit and instead marched his MPs through the division lobbies to vote for it.

Echoing the intervention of Blair on Friday, he said: “Let’s be frank, and I do say this with terrible sadness, the debilitation of our own party contributed to Brexit. We have a leader who, unlike the vast majority of Labour members including many of those who joined up in order to support him, has never been a European true believer.

“And in the referendum he failed the key test of democratic politics, which is to cut through media cynicism and the mass of seething public discontents with a compelling and positive case for Europe which forced voters to listen.

“And now I see no clarion call for the fight, only a three-line whip in the Commons to force Labour MPs to troop through the lobbies alongside a rightwing Tory government dancing to Iain Duncan Smith’s tune.”

Another peer, David Triesman, a former Labour minister, said: “Keir Starmer [the shadow Brexit secretary] has done a fine job, a heroic job in many ways, but I don’t think anybody could say that opposition has been shown fully. I think, for example, the prime minister probably came here yesterday in order to seek a nostalgic reminder of what opposition was like.”

Spencer Livermore, a Labour peer and former adviser to Ed Miliband, also criticised his own party for backing Brexit.

“I have great humility about the outcome of the referendum and about the unelected nature of this house, but if we sincerely believe the course we are on will do untold damage to our country we have a duty whether elected or unelected to say so, to oppose it and to tell the truth,” he said. “I believe that working people’s lives will be made worse by this bill.”

The discontent in the House of Lords comes amid jitters within Labour about the party’s prospects of holding the Copeland seat - one of the two byelections on Thursday.

Some Labour MPs said they were increasingly concerned about the effects of Storm Doris on turnout and those that had visited the seats said the Tory campaigns were growing in confidence.

“Turnout is going to be the problem for us. I think people on a cold day in February will think: ‘Oh, why should I bother?’” one Labour MP said. “The Tories were only a handful of votes behind Ukip and I think they could even win in Stoke. If you’re an anti-Labour voter, and they are the majority in Stoke, you could decide to vote Tory. It could happen with a low turnout.”

However, another Labour source said a low turnout did not necessarily mean there would be a Conservative victory in Cumbria.

“The Tories need people to turn out just as much as we do,” the source said. “We have a very strong ground operation up there. It is genuinely impressive and disciplined considering the geography. The messaging is simply: we are committed to nuclear power and focused on saving the maternity services.”

Another Labour backbencher said they believed the party’s relentless focus on the NHS would see them clinch Copeland. “I am getting the sense that the NHS has really punched through, as indeed it should. Campaign issues-wise, this is almost the perfect byelection for us.”