Opposition parties have rounded on Alex Salmond after it emerged he personally had a hand in delaying embarrassing details about his doomed plans for a local income tax.

Newly released documents show that such was the sensitivity in Government circles over the issue last year the First Minister personally meddled in the wording of a press release on the eventual climb-down.

Last night the Scottish Government insisted everything had been done by the book and the protection of the right to limit the release of material on internal policy formulation was standard practice.

However, Labour's business manager Paul Martin called it a "grubby affair", saying that for the First Minister to be reduced to writing his own press released demeaned his office.

A year ago the Government spent more than £100,000 fighting a Freedom of Information (FoI) disclosure about plans for a local income tax which had the effect of delaying it beyond the Holyrood election in May.

Having fought the FoI case for almost two years, ministers sanctioned a Court of Session appeal which had the effect of kicking the issue into the long grass beyond the election.

This was done during the "purdah" period before the election, leading to more complaints of an abuse of power, although the head of the Civil Service in Scotland ruled that nothing was amiss.

Then in July the Government dropped their case, by which time the politics had moved on and the focus was on the unfolding phone-hacking scandal in London.

Ken Macintosh, Scottish Labour's finance spokesman, said: "How can the people of Scotland have any faith in the First Minister and his case for separation if he goes to these lengths to cover up the numbers if they don't suit him?

"Scotland deserves an honest and open debate about our future, whether it be on the SNP's tax hikes or their faltering case for separation.

"But the First Minister seems determined to play by his own rules."

Liberal Democrat leader Willie Rennie said the whole issue highlighted the cynical attitude to power at the heart of the SNP administration, adding: "These revelations show us that Alex Salmond and his top guard were swarming all over this issue. They were determined to hide the truth from the people of Scotland. When they were forced to release the information they did so on a day when everyone was looking elsewhere.

"It's cynical to pick a busy day to bury bad news. Far from being open government, this is a secret government."

For the Scots Tories, Chief Whip John Lamont, MSP, said: "Alex Salmond has once again been caught red handed trying to bury the SNP's totally misguided plans for a local income tax.

"He went to extraordinary lengths to keep this information out of the public domain last time round, and it now appears he has played a major part in burying the release on what was an extraordinarily busy news day.

"It is not befitting of the office of First Minister to hide information from the public in this manner. By trying so hard to bypass any scrutiny of these plans it only shows that Alex Salmond must have had something to hide."

A Scottish Government spokesman said: "Ministers regard this as nonsense. The material was all published on the Scottish Government's website on Wednesday, and simply shows that officials recommended in June 2011 not to continue the appeal, ministers accepted that advice, and this position was announced soon after in July in the normal way.

"Ministers legitimately pursued the appeal, on the basis that ministers and civil servants need private space to encourage free and frank discussion on the development of policies – a position defended by every administration since time immemorial, otherwise there would be no such advice."