Saturday could be the hottest day of the year so far, with temperatures possibly hitting 32C, forecasters have said.
Parts of south east England and East Anglia could see the mercury tip 32C (89.6F), with large parts of Britain basking in glorious sunshine, according to MeteoGroup, the weather division of the Press Association.
And the rest of England is set to see temperatures in the high 20s, albeit with some showers in the west and Wales and cooler temperatures in Scotland and Northern Ireland.
MeteoGroup senior forecaster Paul Mott said: "It will be a close-run thing really. The hottest day so far was July 25, when it reached 31.4C (88.5F) at the Olympic Park in east London. This weekend will almost certainly get to 31C and it may well reach 32C somewhere in the South East or East Anglia tomorrow."
It means a warm start to the Premier League season. Games taking place in London include Arsenal v Sunderland at the Emirates, Queens Park Rangers v Swansea City at Loftus Road and Fulham v Norwich City at Craven Cottage, while West Ham mark their return to the top flight at home to Aston Villa at Upton Park.
It is also set to be a hot third day of the Test match at Lords between England and South Africa. Other events taking place this weekend include the V Festival at Hylands Park, Chelmsford, and Weston Park, Staffordshire, with headline sets from The Stone Roses and The Killers and other acts including Rizzle Kicks, The Enemy, Pixie Lott, Dappy, The Human League and The Proclaimers.
Mr Mott said there will be more unsettled weather in western areas, which on Friday saw up to 40mm (1.5in) of rain in some places. Northern Ireland and Scotland will struggle to top 22C (71F) on Saturday.
Sunday will present a more unsettled picture across much of the UK, he said, with showers spreading northwards over much of the UK. But he said temperatures could still reach 30C (86F) in London and 26C (78.8F) in other parts of the country.
The Met Office said that while it will be mainly dry in the South East, there will be some cloud around and any sunshine will be quite hazy with fog lingering along some coasts. The South East could become oppressively hot over the weekend as warm and humid air spreads up from continental Europe and there may even be a few thundery showers later on Sunday, it said.
Its chief forecaster, Martin Young, said: "While it will be hot in the South East, we're not expecting wall-to-wall sunshine and it will feel quite humid and oppressive over the weekend. As we head into next week, south-westerly winds will push that humid air away to bring fresher conditions, and showers to north- west Britain."