In his Budget speech in July 2015, Chancellor George Osborne said that the "original tax credit system cost £1.1 billion in its first year. This year, that cost has reached £30 billion."

Here is a guide to how much tax credits actually cost.

:: Are the chancellor's numbers correct?

Not exactly. To make a valid comparison, we need to use like-for-like figures. Tax credits were introduced halfway through the 1999/2000 financial year, so the first full year of operation was 2000/01. According to the Department for Work and Pensions, the cost of tax credits and their equivalents in 2000/01 was £7.5 billion. This is £10.5 billion in today's prices, and compares with £29.1 billion in 2014/15.

:: So like-for-like spending on tax credits has gone up by how much?

Around £18.6 billion since 2000/01. That's an increase of 177%.

:: How much of total government spending is going on tax credits?

In 2014/15 it was 3.9%. This is up from 2.0% in 2000/01 - a rise of 95%.

:: Has total government spending on benefits gone up by the same amount?

No. The amount the government spends on benefits and personal tax credits as a share of total expenditure has barely changed over this period, from 26.7% in 2000/01 to 28.2% in 2014/15.

:: Of that amount, who currently gets the most?

Over half is spent on pensioners - around 55% in 2014/15. The other 45% goes to people of working age and children.

:: And have these proportions changed over time?

Yes. The share spent on pensioners has gone up two percentage points since 2000/01, while the share spent on people of working age and children has fallen by the same amount.

:: What are the main tax credits?

There are now two main types: the Working Tax Credit (WTC) and Child Tax Credit (CTC). Money is paid directly into the claimant's bank or post office account, and is calculated according to circumstances and income. Working Tax Credit can be claimed by people who work a particular number of hours a week but whose income is below a certain level. A minimum amount of Child Tax Credit is payable to all individuals or couples with children up to a certain income.

:: How has spending on these two types of credit changed?

Working Tax Credit and Child Tax Credit were costed separately for the first time only in 2003/04. In that year the combined spending on WTC and CTC in today's prices was £16.9 billion. The figure for 2014/15 is £29.0 billion. This is an increase of £12.1 billion.

:: So government spending just on the current system of tax credits has risen by how much?

Just under three-quarters (72%).