Michael Fallon, the Defence Secretary, has provided the most bizarre defence yet of David Cameron and Downing Street taking five attempts before admitting the Prime Minister had benefited from the offshore fund created by his father, impying it was journalists’ fault for failing to ask the right questions.

That, Mr Fallon suggested, was why it took Mr Cameron until Thursday – four days after the story of the Panama Papers leak first emerged – before he told ITV’s Robert Peston that in 2010 he had made a £19,003 profit from shares in Blairmore Holdings, the fund created by his father Ian.

Speaking on BBC Radio 4’s Any Questions, the Conservative MP said: “I think it was a very difficult issue for the Prime Minister because it concerned his late father whom he much revered.

"Of course it's true that if the fuller questions that were asked of him [on Thursday] had been asked of him earlier in the week then it might have all been neater. But that is hindsight.”

Returning to a more conventional defence of Mr Cameron, he added: “We know three things: first he had shares in this unit trust, which an awful lot of people in this country ... have through their private pensions and you pay taxes on those shares.

"Secondly, we know that he sold those shares before he became Prime Minister back in 2010.

"Thirdly we know that he paid all the taxes that were due when he sold those particular shares. I don't see that he has done anything at all improper."

Mr Fallon’s 'failure to ask the right questions' argument was immediately subject to mockery on social media, some of it coming from the journalists who had spent some of the week asking, or trying to ask the Prime Minister about Blairmore.

The Telegraph parliamentary sketchwriter Michael Deacon appeared to parody Mr Fallon’s argument by writing: “It's all your fault he didn't tell the truth. He desperately wanted to, but you wouldn't let him."

Mr Deacon added: “’Fuller questions’. Before his Peston interview, Cameron deigned to take only one - ONE - question on his tax affairs all week.”

Sky’s Faisal Islam tweeted: “Is this satire? Did he actually say that?”

On Tuesday afternoon, Mr Islam had asked Mr Cameron to “clarify for the record that you and your family have not derived any benefit in the past and will not in the future from the offshore Blairmore Holdings fund".

In reply, the Prime Minister made no mention of his family. Nor did he address the issue of whether he might have benefited from Blairmore in the past – something that two days later he admitted had indeed happened.

Instead Mr Cameron restricted himself to his individual affairs and to the present, saying: “In terms of my own financial affairs. I have no shares, no offshore trusts, no offshore funds, nothing like that.”

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After the Panama Papers leak emerged on Sunday evening, reporters asked on Monday morning whether the Cameron family still had money in Blairmore. A Downing Street spokesman said it was “a private matter.”

On Tuesday afternoon, Mr Cameron gave his reply to Faisal Islam. About three hours later, a Downing Street spokesman said: “To be clear, the Prime Minister, his wife and their children do not benefit from any offshore funds.”

On Wednesday morning a Downing Street spokesman said: “There are no offshore funds which the Prime Minister, Mrs Cameron or their children will benefit from in the future.”

On Thursday evening, Mr Cameron said he and his wife had benefited from Blairmore in the past.

Mr Fallon’s remarks come after the Tory MP Mark Pritchard urged against discriminating against the Prime Minister “just because he was born into a wealthy family”. He also claimed that Ian Cameron had been the victim of “a real injustice.”

Mr Prichard, the MP for the Wrekin, told the BBC: “I think it’s very unseemly, very distasteful and I think there’s a real injustice. It’s an injustice that the Prime Minister’s late father, who did the best for his family, a loving father, a loving son, is not here to defend himself.

“The Prime Minister has been very open. I don’t think he should be discriminated against just because he was born into a wealthy family.”

The Labour leader Jeremy Corbyn has demanded that Mr Cameron make a statement to Parliament on Monday to give a "full account of all his private financial dealings", claiming the revelations raised questions about "personal integrity".

The Labour MP John Mann, who has called for the Prime Minister to resign, said he would ask the parliamentary standards commissioner, Kathryn Hudson, to examine whether Mr Cameron should have declared his profits from the sale of his trust shares in the Commons register of interests.

A Downing Street spokesman said: “The Prime Minister’s interests have always been recorded in line with the rules as they stood at the time.”

Mr Cameron also told ITV News it was a “fundamental misconception” that Blairmore Holdings was established to avoid tax. The fund was created – entirely legally – in 1982.

The continuing UK controversy came as authorities in El Salvador raided the offices of what is thought to have been a local affiliate of Mossack Fonseca, the Panamanian law firm whose documents were leaked.

El Salvador’s Attorney general Douglas Melendez, who personally oversaw the raid, said the government decided to search the offices on Friday after noticing the affiliate had removed its office sign late on Thursday, which raised suspicions. The El Salvador authorities seized about 20 computers, some documents and interviewed seven employees, but did not detain anyone.

An employee later said the company was planning to move, according to the attorney general’s Twitter feed.

Mr Melendez told reporters: “At this moment we cannot speak about (any) crimes; all we can do at this moment is our job.”

He said it appears the law firm’s local affiliate helped process information for clients worldwide.

The El Salvador office is not listed on Mossack Fonseca’s corporate website, and officials from Mossack Fonseca in Panama were not immediately available to comment.