The former Conservative leader William Hague has urged his party to reject proposals to give its members more choice in future leadership contests.

He said the idea, favoured by supporters of Boris Johnson, could give undue influence to “unrepresentative minorities” and reduce the chances of the party selecting a leader with wide appeal.

Lord Hague, who now sits in the House of Lords, also claimed the Labour party – by giving members and supporters a maximum role in the leadership contest, leading to the election of Jeremy Corbyn – had undermined democracy by leaving the UK without a “moderate, easily electable alternative to the government of the day”.

The Guardian reported on Monday that Tory backbenchers were warning of a risk of entryism in the party as Leave.EU encouraged its supporters to become members in order to back Johnson or Jacob Rees-Mogg in a future leadership contest.

Hague was elected Tory leader in 1997 under the old rules that meant he was elected by only MPs. He replaced that with a system that allowed members to select the leader from a shortlist of two drawn up after MPs voted to eliminate other contenders.

Writing in the Daily Telegraph, Hague said that in some respects he was “spectacularly wrong” to have brought in the new system, first used to elect Iain Duncan Smith in 2001.

“I believed at the time that giving a vote to members would help to enlarge the membership and make it more representative of the country, and aimed for a million members of a revived grassroots organisation,” he said.

“The sad reality is that, since then, the total number of Conservative members has halved, and earlier this year officially stood at 124,000. Most of these are wonderful people … But they are often the first to point out that they are not remotely representative of society at large or even of their own voters.”

There is increasing interest in the Conservative party leadership election rules because of speculation that Theresa May may face a challenge in the autumn.

Johnson, who resigned as foreign secretary in opposition to her Chequers Brexit plan, is favourite among Conservative members for the next leader, but he is widely distrusted by MPs and it is thought that were he to stand, he would struggle to make it on to the shortlist for the members’ ballot.

Recently the Campaign for Conservative Democracy (CCD) said any candidate with the support of 20 MPs should be allowed on the final ballot paper, but the CCD is a fringe group with little following in the party and the proposal has not received support from senior figures.

Without commenting directly on Johnson’s possible candidacy, Hague said it would be a mistake to change the rules in the hope of benefiting an individual.

He explained: “Calculations of this kind are often wide of the mark or counterproductive, and most Conservative leadership battles have sprung a major surprise. In any case, short-term needs make poor long-term rules.”

Labour allows any candidate with the backing of 10% of MPs and MEPs (down from 15% at the time of Corbyn’s election) to be included on the final ballot, with voting open to all members and registered supporters.

Hague claimed in his article that Corbyn would never have been elected leader under the Conservative rules and that he was “grossly unrepresentative” of Labour MPs and the public at large.

But Corbyn’s election has led to the party having more than 500,000 members, and at the last general election claims that Corbyn’s radicalism would ruin Labour’s chances turned out to be unfounded, with the party gaining votes and seats.