The package - described as "reform for the medium term" - will include changes to benefits for disabled people.

It is expected to build on the groundwork put in by former social security minister Frank Field's much-hyped Green Paper on the subject, published earlier this year.

Mr Darling will make a statement outlining his proposals in the Commons at 3.30pm (1530 GMT) after Prime Minister's Questions.

Frank Field: Resigned at the last reshuffle when refused the top job

The prime minister's official spokesman said Mr Darling's announcement would put to rest suggestions that the government's reform programme has run into the sand following the departure of Mr Field and the sacking of his boss, Harriet Harman, at the summer reshuffle.

Mr Field left the government when Tony Blair refused his request to make him Social Security Secretary in Ms Harman's place.

Mr Blair's spokesman also confirmed that a welfare reform bill will be in the Queen's Speech next month, and said the underlying principle of the changes to be announced is "work for those who can, security for those who cannot".

The spokesman denied Mr Darling's statement had been brought forward a day to deflect attention from Welsh Secretary Ron Davies' high-profile resignation.

The social security secretary's statement to MPs will be accompanied by the release of three documents.

One will set out an overview of progress to date and the principles underlying the government's approach to changing the welfare system.

The second will be a consultation paper on support for disabled people.

And the third will discuss the "single gateway" proposed for all benefit claimants, providing a "one-stop shop" for them to get benefits and help with finding work.

Proposals on pensions and widows' benefits are expected to follow at some stage in the future.

Signalling a likely increase in means-testing, Mr Blair's spokesman said the proposals were about "more help and more money for the poorest people and the most severely disabled who are the moment get a terrible deal from the system".

But he added that the government was likely to adopt the same approach as it did when it cut benefits for lone parents, but applied the reduced payments to new claimants: "All existing claimants will be protected. This is reform for the medium term."

Doing it this way would permits the government to claim, as it did over the lone parent benefit cut, that no individual will suffer a drop in their benefit.