Tony Blair is to go to the House of Commons to explain his controversial cabinet reshuffle to MPs.
There is a storm over abolition plans for the lord chancellor
News of Wednesday's statement comes after the Conservatives accused Mr Blair of "tearing up" 1,400 years of British constitutional history "in a single press release".
The government is to scrap the historic post of lord chancellor and create a supreme court, but critics say he should have consulted on the plans first.
Asked if the prime minister was being forced to make the statement, Downing Street Mr Blair was "perfectly happy" to explain his decision.
Later, Commons Speaker Michael Martin said he was "concerned" about the changes and so had asked for the statement.
It is highly unusual for the speaker to make such a request, or at least to reveal it publicly.
Speaker Michael Martin unusually made his request to Blair public
The Tories are now making the constitutional upheaval the subject of their opposition day debate on Tuesday.
In a letter to Mr Blair, Tory leader Iain Duncan Smith said he presumed the prime minister would speak for the government.
He said it was "unsatisfactory" for MPs to have to wait until Wednesday for Mr Blair's statement as "urgent issues remain unanswered".
But Mr Blair is not rising to that challenge, saying he has a long-planned lunch appointment with Pakistan's President Pervez Musharraf followed by an afternoon speech on public services.
Leader of the House Peter Hain will represent the
government instead, with his opposite number, Eric Forth, leading the debate for the Tories.
In response, Mr Duncan Smith said: "Tony Blair is running scared in face of the chaos and confusion sparked by
his botched reshuffle."
He added: "The prime minister should stop hiding away in Downing Street and come to the
Commons as soon as possible to explain himself."
PLANNED CHANGES INCLUDE
Supreme court to replace the law lords
Independent body to appoint judges
Abolition of lord chancellor's role (criticised as being both judge and cabinet minister)
New department of constitutional affairs - but not a full Justice Ministry
There is much support among many lawyers and MPs for much of the legal changes, but consternation about the lack of notice given.
Criticism and confusion over the reshuffle has also centred on the fate of the Scotland and Wales Offices, which are now part of the newly created Department of the Constitutional Affairs.
Mr Blair's spokesman urged people to look at the substance of a "radical and far-reaching reform" of the judiciary.
The Westminster criticism of the changes would die down, he predicted.
Before news of the statement, shadow chancellor Michael Howard said the confusion surrounding the changes was more than a problem over presentation.
"The trouble here is they do not know what they are doing," he said.
The post of lord chancellor and the Wales and Scotland Offices had been abolished and then reinstated, he argued.
"It's an absolute scandal and the prime minister should come to the Commons and explain it, if he can, this afternoon," Mr Howard told BBC News 24 on Monday.
He added: "We had the most extraordinary thing happen last week - 1,400 years of our history was torn up in a single press release.
"This was done without any consultation, without any preparation and as far as I can see without any thought at all."
Last week's long-awaited reshuffle saw the surprise resignation of Health Secretary Alan Milburn, who says he wants to spend more time with his young family.
Lord Chancellor Lord Irvine retired from the cabinet, but there have been reports he was in fact sacked.
Also on Monday, Lord Williams of Mostyn, the Leader of the House of Lords, said he would consult on changing the system of appointing a speaker for the Lords.
Traditionally, the lord chancellor is the speaker in the House of Lords.
Lord Falconer, who assumes the lord chancellor's duties for the moment, says the changes will "embed" the independence of judges.