Tony Blair will be quizzed by the inquiry on Friday
The Lib Dems and Tories are calling for key documents on the Iraq war to be published ahead of Tony Blair's appearance before the Chilcot inquiry.
The Lib Dems have accused the government of trying to "gag" the inquiry by refusing to publish them.
The documents include letters between Mr Blair and President Bush.
The Cabinet Office said no documents had been withheld from the inquiry but some needed legal clearance before they could be released to the public.
'Complex legal issues'
A spokesman said: "The inquiry has made a number of requests to the government for the declassification of information.
"The government has been working closely with the inquiry to prioritise these requests, consistent with the inquiry's schedule of public hearings.
"Many documents have been declassified and the inquiry has made these publicly available on its website.
"In some instances, the government has required further time to consider the inquiry's requests.
"This may be to allow the government to consult a wide range of parties who provided the original information, or to take further advice.
"In some cases, technical and complex legal issues regarding the law officer's advice or the principle of legal professional privilege are raised, on which we may require further advice.
"In these difficult cases, the government has communicated to the inquiry that it requires more time to give a view on declassification."
Inquiry chairman Sir John Chilcot and ex-Attorney General Lord Goldsmith have both called for publication of the documents.
Lord Goldsmith, giving evidence on his advice about the war's legality, said he did not agree with the decision not to declassify all the documents.
Sir John added: "Can I just say that the frustration is shared."
Mr Clegg said he also shared their "frustration" and warned that failure to release the documents could damage the credibility of the inquiry in the public's eyes.
"Despite Gordon Brown's claim that he has 'nothing to hide' this has all the hallmarks of a cover up.
"Just as Liberal Democrats warned, the protocol on the release of documents is being used to gag the inquiry.
"To restore trust in the inquiry the government must immediately declassify certain key documents ahead of Tony Blair's hearing - the memo from Sir David Manning to Tony Blair dated 31 January, 2003 and the letter from Tony Blair to George W Bush sent July 2002."
He said that if former prime minister Tony Blair, who will face the inquiry on Friday "gets through on the nod due to the withholding of key documents, the public will rightly dismiss this inquiry as a whitewash".
Shadow Foreign Secretary William Hague has also called for the documents to be published.