Media playback is unsupported on your device Media caption Shadow chancellor John McDonnell makes energy price pledge on BBC Breakfast

Labour has promised to place a cap on the prices charged by energy companies if it wins power.

Shadow chancellor John McDonnell made the commitment to the BBC a day after Npower said it was raising electricity prices by 15% from March.

Meanwhile, Mr McDonnell used a speech earlier to vow to "close the gap" between spending in the north and south of England.

But the Conservatives said Labour's plans would "crash the economy".

Npower said on Friday that its price rises would take effect on 16 March, with gas increasing by 4.8%.

The company said the changes would only affect about half of its customers, but energy regulator Ofgem said it must "justify" why it was introducing one of the largest price rises in years.

Speaking on BBC Breakfast, Mr McDonnell said people would be paying for the increase having had their wages "frozen" since the recession.

Asked what Labour would do, he said it would introduce legislation that would limit the amount prices could increase by.

"This is huge. We can't allow this to keep going on," he said.

'Rest of the country'

Speaking later at Labour's economic conference in Liverpool, Mr McDonnell said the North had consistently lost out in terms of spending - and promised a future Labour government would try to rectify that.

He cited figures from the National Infrastructure Commission suggesting projected capital spending per head of population in the north of England would increase by just £1,491 by 2020-2021, compared with by £3,114 per head in London.

"We have to put an end to the Whitehall view that what's good for the City of London is good for the country as a whole," he said.

"It is time for the rest of the country to get a look-in."

Pledging to create a "Crossrail for the North", Mr McDonnell said the high-speed line would reduce journey times between Manchester, Liverpool and Leeds and create 850,000 additional jobs by 2050.

The proposed HS3 line was approved in the government's March 2016 Budget but Labour says the government has still not brought forward a definite plan for the scheme.

Image copyright Reuters Image caption The new Elizabeth Line in London will be fully open by the end of 2019

Mr McDonnell also said a Labour government would pass laws requiring ministers to audit capital spending in the English regions against their relative economic need and to report any disparities to Parliament.

Rail and road investment

The government has earmarked £13bn on transport in the North and £19bn in total capital spending for the region.

It has created Transport for the North - a new body to co-ordinate transport investment in the region, modelled on Transport for London - and has accepted all its recommendations to improve connectivity.

Its plans include an accelerated upgrade of the M62, major improvements to junctions on the M1 and M56, investments in leading stations and £1.2bn to increase capacity on the Northern and TransPennine rail routes.

A Conservative spokesman said Labour's borrowing plans "would crash our economy and threaten jobs and infrastructure".