Charities have failed to speak out against the Conservatives’ manifesto plans for social care because they are afraid of falling foul of the lobbying act, according to Sir Stephen Bubb.

Bubb, director of the think tank Charity Futures, said the lobbying act was gagging charities, but he also criticised charities for allowing themselves to be silenced by it and called on sector leaders to “get a backbone”.

The Conservative Party has been criticised for the social care proposals in its manifesto, such as the inclusion of the value of elderly people’s homes when estimating how much they should contribute to their own care. But Bubb said not enough of this criticism had come from charities.

“The social care proposals strike at the heart of what charities do and they should be up in arms about them, but it hasn’t happened,” said Bubb, who was until last year head of the charity leaders body Acevo.

“It is two problems: there is the problem of the so-called ‘gagging act’, but also the general climate of hostility towards charities means there is a lot of self-censorship,” he said.

“Charities that once would have spoken out are keeping quiet and doing a disservice to their beneficiaries. They need to get a bit of a grip.”

The Transparency of Lobbying, Non-party Campaigning and Trade Union Administration Act 2014 introduced caps on the amounts that organisations – including charities – can spend on campaigning on issues that could be perceived as party political in the run-up to elections.

Both Labour and the Scottish National Party have pledged in their general election manifestos to repeal the parts of the act that relate to charity campaigning.

“Many charity leaders do feel that if they do speak out there will be some form of comeback on them,” Bubb said. But this response was “simply not good enough”, he added.

“The deafening silence from charities in the last few months has been extraordinary, and the bottom line is that we are letting our beneficiaries down,” he said. “Particularly the health and social care sector: it needs sorting and charities should be at the forefront of demanding action and answers.

“It is not enough to hunker down and keep their heads below the parapet. Some sector leaders need to get a backbone: that’s what they’re being paid for.”

Responding to Bubb’s comments, Steve Clapperton, campaigns manager at the Charities Aid Foundation, said the idea that some charities had felt unable to speak out on behalf of their beneficiaries during the general election campaign was “extremely concerning”.

He said: “CAF has consistently warned that the lobbying act might deter charities from fulfilling their important role in our democracy.

“Concern is building among charities and politicians, including some MPs who initially supported the act, who have noted the absence of charities in policy debates where they would normally represent expert views on behalf of their often marginalised beneficiaries.”

He said it was “legitimate and vital that charities were able to positively influence political parties’ policies and called on all parties to come together after the election and commit to repealing the lobbying act.

Elizabeth Chamberlain, head of policy and public services at the National Council for Voluntary Organisations, said: “Charities can, and should, campaign in the run-up to an election.

“The need to maintain their neutrality becomes greater when an election is called, but it doesn’t mean that charities lose their right – and duty – to speak up on behalf of their beneficiaries and about their cause.”

But she echoed Bubb’s concerns about self-censorship, which she said was the biggest threat to charities’ right to campaign.