LONDON, Sept 21 (Reuters) - The British government on Wednesday said it would cap wholesale electricity and gas costs for businesses at less than half the market rate from next month, helping relieve the pressure of soaring energy costs but adding to the government's fast-rising spending.

The government did not publish any estimate of the cost, but Citi forecast 25 billion-30 billion pounds over the next six months and other reports have put the price at up to 42 billion pounds.

That comes on top of more than 100 billion pounds for a previously announced scheme to help households.

A spokeswoman for Prime Minister Liz Truss said Kwarteng's fiscal event would outline estimates of the cost of the support packages for business and households.

On Friday, Kwarteng is expected to set out some detail on how he will pay for the energy scheme while at the same time delivering on promises to cut taxes, although the total cost of the energy scheme will depend on market prices over the coming months.

The business energy scheme will initially apply from Oct.

The scheme initially gives urgent support to all such institutions that may need it, but is expected to be narrowed down in March with support targeted to the ones that most need it.

The government also announced support for households in Northern Ireland on the same level as the equivalent scheme in the rest of the United Kingdom, taking effect from November but backdated to the start of October.

The scheme was heavily criticised by most political parties in the region who said the 100 pound additional payment to households unable to receive support through the price cap because they use alternative fuels such as heating oil was insufficient.