The British Pregnancy Advisory Service (Bpas) said women were risking prosecution by using the pills to induce an abortion, but most were unaware of the risk.
Bpas, which provides abortions at its clinics across the country, obtained data from the Medicines and Healthcare products Regulatory Agency (MHRA) which shows 645 abortion pills were seized in 2015 and 2016 on their way to addresses in Scotland, England and Wales.
The number of pills seized as part of Operation Pangea has risen from five pills in 2013 to 375 in 2016.
Bpas, which wants abortion to be decriminalised, said many women were unaware they could face prosecution and lifetime imprisonment for buying pills online to induce their own abortion.
The law states that abortions can be provided only when women meet certain requirements and two doctors approve the request.
Ann Furedi, chief executive of Bpas, said: “At Bpas, we do all that we can to make abortion services as accessible as possible. However, it is clear that for some women the barriers to clinic-based treatment feel insurmountable.
“These are women in desperate and difficult circumstances. They are not criminals deserving of life imprisonment.
“Evidence suggests that as awareness of online abortion pills is increasing, so too are the numbers of women using these methods.
“Women in these circumstances should not face criminal punishment and we should not support a legal framework which threatens just that.”
Women on Web, a not-for profit online abortion provider, sends abortion pills to countries where abortion is illegal. It will also send them to women in the UK under exceptional circumstances.
British women contacting Women on the Web show some of the reasons being given for seeking pills online.
One woman wrote: “I’ve just found out I’m pregnant and I can’t keep the baby, can you tell me if I can get the tablets from you please?
“I am in the UK but it’s impossible for me to get to a clinic due to having a disabled daughter who I can’t leave and I have no one else I can trust.
“I’m in a complete mess, clinics said I have to leave my daughter at home but I have no one else at all to have her, due to her disabilities a nursery can’t have her. I’m one week late.
“I’m in good health and have no allergies or medical conditions. Please I’m really desperate for help.”
Another said: “I live in [a rural area in England] and have no friends and the relatives I have I am not close to.
“I was hoping to have a termination in the comfort of my own home without judgmental eyes and without worrying about my husband knowing.
“I fear what would happen if he did. I have three children and my third is 11 months old. I considered an abortion when he was conceived and had a terrible pregnancy and still suffering from post natal depression.
“I will try to seek help, anonymously if possible. I’m in great need of help.”
Sophie Walker, leader of the Women’s Equality Party, which supports decriminalisation, said: “Women’s equality and wider choices depend on having control of our own bodies.
“In 2017, it should not be the case that women still have to fight for their reproductive rights and access to sexual health care.”