There will a period of silence held before the start of the London Marathon and runners are also being encouraged to wear black ribbons after the Boston marathon blasts.
Virgin London Marathon, the organisers, have pledged to donate £2 for every finisher in the race to The One Fund Boston set up to raise money for victims of the explosions.
Roughly 35,500 people are expected to cross the finish line on The Mall, meaning at least £70,000 is likely to be raised.
Tatyana McFadden, who won the women's wheelchair race in the Boston Marathon on Monday, has confirmed she will still take part in the London Marathon and is 'running for Boston':
More top news
There will be sunny spells for many but heavy showers will spread across the UK in the afternoon.
Children's Commissioner review found 28% of children in need of support were sent away because their illness was not deemed serious enough.
The former prime minister suggested a Jeremy Corbyn government would be a 'very dangerous experiment'.