The Institute for Government has questioned the truthfulness of recent Conservative Party claims that a "supermajority" for Labour in next month's general election would be bad for parliamentary scrutiny.
Defence secretary Grant Shapps is a leading proponent of the theory. As the Conservatives have continued to trail Labour by 20 points in the opinion poles ahead of the 4 July vote, Shapps has repeatedly cautioned that a parliamentary landslide for Sir Keir Starmer would be "very dangerous for Britain".
The defence secretary's argument is that Labour would enjoy "unchecked power" with a majority significantly in excess of the 80 seats that then-Conservative leader Boris Johnson secured in 2019's general election.
IfG director Hannah White said it was "untrue" to suggest that a Labour supermajority would lead to a sudden dramatic loss in the effectiveness of parliamentary scrutiny.
"The rules of the House of Commons have always provided significant advantages to the party of government – being one of the most executive-dominated parliaments in the world," she said in a comment piece on the think tank's website.
"Curiously, this is a fact that has not appeared to bother the Conservatives for the past 14 years."
White added that concerns about the impact of any future supermajority in parliament came against the backdrop of declining scrutiny over the past decade.
"In parliamentary terms the difference between an 80-seat and 200-seat majority is not material," she said. "The most significant factor for democracy is the attitude a government takes to the role of parliament and the value of scrutiny."
Tony Blair's New Labour secured a majority of 179 seats in its landslide 1997 general-election win.
However, White pointed to former prime minister Theresa May's decision not to give parliament a meaningful role in discussions on what shape a Brexit deal should take during her three years in No.10 as an example of why a supermajority need not be an issue.
"Even with a slim and then non-existent majority she was able to proceed – ultimately to her own detriment – without allowing meaningful scrutiny of her plans, at one stage refusing to allow inconvenient opposition or backbench debates in the Commons for a period of over five months," White said.
She added that May's successor, Boris Johnson, had used the 80-seat majority he secured in 2019 to adopt the attitude that nobody should be able to oppose his parliamentary plans.
"His ministers refused to entertain even the smallest amendments to bills and some were extremely reluctant to appear before select committees," White said. "He was free with the creation of new ministerial powers and their exercise – using secondary legislation to pass numerous measures without the possibility of parliamentary opposition. In practice, his opposition came more from within his own party than other parties."
White said that voters who are worried about the prospect of a large Labour majority should grill Labour candidates about the attitude that they plan to take to scrutiny in government.
She acknowledged, however, that there would be significant downsides of a Labour supermajority for the Conservative Party, over and above the loss of power.
She said shadowing government ministers and chairing and sitting on select committees would be "challenging" for a Conservative official opposition "much outnumbered" by a Labour government.
White added that the payment of "short money" funding to support the official opposition relies heavily on the number of seats won, meaning that a "much diminished" opposition would also face an additional financial crunch.