Betty Boothroyd has claimed that Labour will be “galloping towards the precipice” if it elects Jeremy Corbyn as its leader.

In the latest intervention from a major Labour figure concerned at the rise of the leftwing leadership candidate, the former Speaker of the Commons said the party could be heading for the “scrapheap of history”.

Lady Boothroyd’s comments came as Sunday morning’s headlines show the acrimonious leadership fight is facing further claims of internal party chaos, voter infiltration and threats of legal action.

Boothroyd, the first female Speaker who fought the Trotskyite group Militant in the 1980s, accused Corbyn’s hard-left supporters of “deluding a new generation” with the same extremist “claptrap” that gripped the party 30 years ago.

Writing in the Sunday Times, she blamed Ed Miliband, the former leader who created the new voting rules, saying his “decision to abandon the leadership only hours after the party’s defeat in May started the “rot” and was an “act of self-indulgence”.

Boothroyd wrote: “If Labour ends up on the scrapheap of history, it will do so because of its own foolishness and self-inflicted wounds. What party in its right mind would allow a combination of far-left enemies, militant trade unions and first-time supporters to decide its fate?”

She also condemned Corbyn for sharing a platform with Tony Mulhearn, a Militant “bully boy” from Liverpool Boothroyd helped to expel from the Labour party in 1986.

“Mr Mulhearn and his Trotskyite comrades nearly bankrupted Liverpool in the Eighties. We cleaned the stables. They may need cleaning again,” she wrote.

Alastair Campbell, Peter Mandelson, Gordon Brown, David Miliband, Tony Blair and David Blunkett have all previously called for party members to vote against Corbyn.

Boothroyd, who sits as a crossbench peer after giving up her party allegiance to become Speaker, said: “My old party is galloping towards the precipice. I urge it to heed the jagged rocks before it is too late.”

Some senior Labour figures have predicted that 50,000 of those who have signed up to vote in the election will not be vetted. Former ministers, party donors and senior activists are reportedly in talks about mounting a legal challenge to the result on the grounds that the vetting of voters is “a shambles”.

Former frontrunner Andy Burnham, in an interview with the Independent on Sunday, he appeared to rule himself out of any court action should Corbyn win.

More than 120,000 people have signed up to vote, along with more than 189,000 members of unions and other affiliates, bringing the electorate to more than 600,000.

With 12 working days left before voting closes on 10 September, sources have claimed that fewer than 5,000 entries a day are being processed. Both Conservatives and leftwing “entryists” have been accused of joining the Labour party to vote for Corbyn.

New regulations introduced by Miliband allowed members of the public to sign up to vote as a registered supporter for £3.

Lord Falconer, a former lord chancellor and a serving QC, has warned Harriet Harman, the acting Labour leader, that the party is vulnerable because Corbyn’s supporters have blocked officials from using data gathered during election canvassing to see if they supported other parties in May.

Party officials held crisis talks on Saturday and further talks between the campaigns will take place on Tuesday.

Former ministers and donors are also preparing to go to court to challenge the party’s decision not to release a breakdown after 10 September of how full party members, union affiliate members and registered supporters voted.

Speaking on BBC Radio 4’s Any Questions on Saturday, Corbyn rejected the suggestion it had been a “summer of blood” for Labour and said the campaign had been “good for democracy and for the party”.