Conservative party membership has almost halved since David Cameron became leader, it was confirmed, as party chiefs bowed to pressure to reveal the latest figure.

There are now 134,000 constituency members, down from the 253,600 who voted in the 2005 leadership contest – a drop of 47% – the ConservativeHome website was told.

The party's previous refusal to publish its membership numbers fuelled speculation that it had fallen below 100,000, amid serious concerns over its ability to campaign in key seats in 2015.

The UK prime minister has been making efforts to reassure grassroots Tories about his leadership, which has been marked by significant dissent over issues including Europe and gay marriage.

Senior constituency figures have warned of an exodus of members to the benefit of the eurosceptic UK Independence party, which has been performing strongly in elections and opinion polls.

Former MP Paul Goodman, who spearheaded the ConservativeHome campaign for the figure to be published, suggested it was part of a move towards more openness on the issue.

The Tory party chairman, Grant Shapps, said he wanted to send MPs a league table of their own constituency's standing, which would "concentrate a few minds".

Labour called the dramatic drop "a damning indictment of Cameron's out-of-touch and weak leadership" and said its own membership had risen under Ed Miliband.

The Labour vice-chairman, Michael Dugher, said: "The Tories are deserting Cameron in their droves. Under Cameron, the Tories are becoming an endangered species. They are getting fewer and fewer in number by the day – no wonder they stand up for the few and not the many.

"Labour has increased our membership under Ed Miliband's leadership. And now Ed Miliband is absolutely determined to keep changing Labour, to open up politics more to the public, and to ensure that Labour is a genuinely one nation party that is rooted in every community."

The Conservatives boasted as many as 3 million paid members in the early 1950s but there has been a dramatic reduction across all parties in subsequent decades.