Nick Clegg has been accused of ignoring alleged sex abuse victims of the late Liberal Democrat MP Cyril Smith by failing to respond to formal requests for help.

The deputy prime minister has been sent two letters by a solicitor's firm that represents four men who say they were attacked in the 1960s and 70s by the MP for Rochdale, who died in 2010.

The letters, the first of which was sent seven weeks ago, asked for the release of files from the Liberal Democrat and former Liberal chief whips' offices following the suspicion that Smith's propensity to abuse was common knowledge among senior politicians. Police believe he was a prolific abuser of boys and should have been charged more than 40 years ago.

The disclosure will increase pressure on the Lib Dems over claims that they have ignored allegations of sexual harassment and assault within the party.

Last week, they were forced to confront allegations of sexual harassment against Lord Rennard, one of the party's most senior figures. He denies any wrongdoing.

Alan Collins, a partner in the law firm Pannone, said Clegg had failed to acknowledge the letters. "We have formally written to Nick Clegg's office on two occasions since 3 January requesting the chief whips' files on the basis of our clients' instructions and a CPS report into allegations of abuse at a hostel in Rochdale," he said.

"We have good reason to believe that they contain information which will show that senior figures in the party were well aware of the allegations against Smith. However, we have not yet received a reply from the deputy prime minister."

Collins has also written to Theresa May, the home secretary, asking for files from the security services to be released.

The Lib Dems' chief whip, Alistair Carmichael, conducted an inquiry into the parliamentary party after the allegations against Smith arose in November. He concluded that senior party figures did not suspect that Smith was a child abuser.

But the former Liberal party leader Lord Steel, when asked by the Guardian, said he had heard rumours that Smith used to deliver punishment beatings to boys.

The Guardian disclosed on Wednesday that Jeremy Thorpe, another ex-Liberal leader, was among party grandees who knew about the allegations in the 70s but failed to launch a formal inquiry, according to a Lib Dem parliamentary candidate.

Clegg has also been accused of ignoring requests to investigate allegations against Mike Hancock, the MP for Portsmouth South, who is being sued for allegedly sexually assaulting a constituent who had approached him for help with noisy neighbours. Hancock denies any wrongdoing. Clegg was sent a complaint in March 2011 but there was no reply or investigation, it is alleged in this week's Spectator.

The party announced an inquiry last week into how it has handled past complaints of sexual impropriety. Tim Farron, the Lib Dem president, has admitted that the party "screwed up" over inquiries into claims that Rennard groped or propositioned female activists.

Smith was named in November on the floor of the House of Commons as a serial abuser of boys. Victims of Smith claim he abused many young boys in a hostel and a school in the late 1960s and continued to abuse others into the 1980s.

Police first investigated the claims in 1968, but the Crown Prosecution Service concluded there was no case to answer.

In November, the CPS re-examined their files but this time said that, if the same evidence was unearthed today, they would have prosecuted Smith.

A spokesman for the Lib Dems said: "We have not refused to provide files. We have no files to provide, and we will be responding formally to that effect shortly."

A Lib Dem source said: "There's absolutely no question of anybody withholding anything. It's taken a bit longer to reply than we hoped but that's because we wanted to do as thorough job as possible."