Temperatures in the south of England could break the 20C (68F) mark this week, forecasters said.
But as the South West, South East and East Anglia bask in the spring sunshine amid unseasonably warm conditions, the rest of the UK will experience average temperatures and some rain.
Experts said the North-South split was down to a frontal system.
Brendan Jones, a forecaster for MeteoGroup, the weather division of the Press Association, said: "For the south of the country the warm weather is going to hang around on Tuesday, Wednesday and Thursday, but it's certainly fairly cool for the north of the country."
He said the mercury was set to rise to 19C or 20C (66F-68F), and possibly as high as 22C (71F) in some parts - far higher than the average of 13C or 14C (55F-57F) for late April.
In Scotland and Northern Ireland, however, it will stick to the average of between 9C and 12C (48F-53F), while in the north of England and the Midlands there could be highs of 16C (60F).
But on Friday, temperatures are likely to drop across the UK.
"It does look as if it's going to turn cold from the north everywhere," Mr Jones said.