Explanatory memoranda are also laid alongside the Estimates. These explain the expenditure headings in the Estimates, any changes from previous years, and new areas of spending. They also explain the Barnett formula calculation of the block grant for the devolved governments.

One of the select committees’ core tasks is to 'examine the expenditure plans, outturn and performance of the department and its arm’s length bodies, and the relationships between spending and delivery of outcomes'. The House of Commons Scrutiny Unit provides select committees with briefing material on each departmental Estimate, as well as support with any follow-up scrutiny, including questioning of ministers or senior departmental officials.

MPs find it difficult to scrutinise the Estimates because the documents are dense, complex and difficult to understand. Expenditure is set out under very broad headings and is rarely linked directly to identifiable projects or programmes.

It is therefore difficult for MPs to focus on particular areas of spending other than in very broad terms, and even more difficult to target particular projects or programmes, should they wish to reduce spending via an amendment to the Estimate.

The presentation of Estimates information in the UK has been criticised by the Organisation for Economic Co-operation and Development (OECD). Some academic observers contend that the high-level aggregation of financial information means that UK government ministers have far more discretion in implementing public spending than do ministers in other OECD countries.

Previously, the Liaison Committee selected topics for Estimates Day debates, based on applications from select committees. Now, however, under an arrangement recommended by the Procedure Committee in April 2017 and then introduced on a pilot basis, any backbench MP - including select committee chairs - can bid, via the Backbench Business Committee, for an Estimates Day debate on one of the departmental Estimates.

The Backbench Business Committee considers all the bids and then submits formal proposals to the Liaison Committee as to the Estimates debates that should take place on the allocated days. Under the power set out in Standing Order No. 145, the Liaison Committee then recommends these proposals to the whole House, which must agree them.

In deciding on the bids made for an Estimates debate, members of the Backbench Business Committee take into account factors such as demonstrable levels of cross-party support, gender balance, and evidence of speaker demand.

Of the three days allotted in the House of Commons for Estimates Day debates, one day is allotted for consideration of the Main Estimate, and two days for the Supplementary Estimates and Vote on Account (if needed). One of the total three days may be taken in the form of two half-days of debate.

An Estimates debate may be linked to a department's full spending programme or a particular aspect of it, and it may draw on a relevant select committee report.