LONDON, July 18 (Reuters) - Britain was braced for temperatures to hit 40 degrees Celsius for the first time this week, with ministers urging the public to stay at home as the heatwave caused train services to be cancelled, one London airport to shut its runway and some schools to close.

Much of Europe is baking in a heatwave that has pushed temperatures into the mid-40s degrees Celsius in some regions, with wildfires raging across tinder-dry countryside in Portugal, Spain and France.

Britain's government triggered a "national emergency" alert as temperatures were forecast to surpass the previous record of 38.7 degrees Celsius in the Cambridge University Botanic Garden in 2019.

On Monday, the temperature nudged 38 degrees Celsius in southern England and a new high was recorded in Wales, according to Britain's Meteorological Office. It is expected to be even hotter on Tuesday, when the 40 degrees Celsius level is forecast to be exceeded.

"We've got a difficult 48 hours coming," Kit Malthouse, a minister in charge of government coordination, told BBC radio.

The heat brought widespread travel problems.

London's Luton Airport said flights were suspended after a surface defect was found on the runway, and the heat caused flights to be diverted from the Royal Air Force's Brize Norton air base.

The national rail network urged passengers not to travel unless necessary, with some services - including a key route between northeastern England and London - not running during parts of Tuesday.

London's metro network imposed temporary speed restrictions, meaning it would run a reduced service with journeys taking longer than normal.

Jake Kelly from Network Rail said he hoped normal operations would resume on Wednesday, when temperatures are forecast to fall, but that would depend on "the damage that the weather does to the infrastructure over the next couple of days".

To highlight the issue, Network Rail tweeted a picture of a rail with a kink near London.