Prime Minister Tony Blair last night fought to regain control of the political agenda with a speech on public service reform, as his "botched and shambolic" reshuffle was being savaged in the Commons.

Mr Blair delivered his familiar message on the need for Labour to rally behind his planned shake-up of Whitehall at the soft-left Fabian Society's annual summer reception, rejecting Tory calls to take part in the Commons debate.

Instead, Mr Blair will make a statement to MPs today, forced on him by parliamentary pressure, on the constitutional changes brought on by his ministerial shake-up.

Downing Street said he was "relaxed" and "more than happy" to explain the downgrading of the posts of Scottish and Welsh secretaries, the potential abolition of the post of Lord Chancellor and the removal to a new Supreme Court of the Law Lords' function as the court of last resort.

But in the Commons, MPs heard shadow Commons leader Eric Forth say: "Last Thursday the Prime Minister's press office announced sweeping change to our constitution and system of justice as a half-baked afterthought to the most botched and shambolic reshuffle in living memory.

"Since then the Prime Minister hasn't deigned to come to the House to explain what it all means."