Almost all of England and most of Wales and Northern Ireland are likely to be hit by thunderstorms on Saturday, forecasters have said. The Met Office has issued a yellow warning for heavy showers and thunder, which could cause flooding, power cuts and travel disruption across much of the UK. The warning covers large expanses of the three nations, with the exception of some coastal areas, and lasts from midday until 10pm on Saturday. This follows the unsettled summer weather that has persisted across the UK throughout August. Much of Scotland, including Glasgow and Edinburgh, is expected to see rain. Forecasters said there was a good chance that spray, standing water and hail would worsen driving conditions in the affected areas, leading to longer journey times by car and bus, as well as possible delays to train services. “Some short-term loss of power and other services is possible, with possible damage due to lightning strikes. Flooding of a few homes and businesses could occur,” the Met Office said. Its meteorologist Simon Partridge said the City of London had had 117% of its usual August rainfall already, with 60mm (2.4in) of rain falling on the capital up to 17 August, compared with 51mm usually seen over the entire month. Northern Ireland has had 90mm of rain – 90% of its 97mm average for the month. Partridge predicted that up to 40mm of rain could hit parts of Northern Ireland and south-west England over 24 hours on Saturday, and the same amount in the northern Pennines on Sunday. Heavy rain and hail have already caused “significant problems” for harvests, according to the National Farmers’ Union. A spokesperson for the union said this year’s harvest was delayed because of cold spring temperatures, but it was too early to say how much profits would be affected and whether consumers should expect a rise in food prices. “The localised heavy rain and hail has caused significant problems in certain areas. It’s a mixed picture because for many areas they just haven’t had many clear dry days in a row to harvest, so things have been a bit ‘stop-start’ with harvesting. Where they have been harvesting, grain has often had to be dried because the better weather hasn’t lasted long enough to get moisture levels down to where they need to be,” the spokesperson said. “The problems will grow if the unsettled weather continues because it will start to impact on the quality of the grain if it goes on too long, especially for crops like milling wheat.”