A bus-load of women from Staffordshire, campaigning to have a breast cancer drug available on the NHS, are handing in a petition at Downing Street.
It is only available for those in the advanced stages of the disease
They will join women from around the country for a meeting with ministers to discuss the drug Herceptin.
People in the early stages of the disease cannot have the drug for free.
Campaigner Barbara Clark, 49, of Bridgwater, said this week she is preparing to sue the NHS for refusing to prescribe it.
Somerset Coast Primary Care Trust said it was operating within national guidelines.
But Ms Clark said she would go to the High Court if the trust did not change its decision by the end of September.
Forty nine people from Staffordshire headed south on Thursday morning for the latest part of their campaign.
They will gather in Parliament Square before marching to Downing Street.