The British Pregnancy Advisory Service (BPAS) says women could find it difficult to obtain the drug quickly over the holiday period.
It is offering emergency contraception free of charge in the post to women who fill in an online form and talk to a nurse over the phone.
An anti-abortion charity said this could encourage "unwise" behaviour.
The Christmas and New Year period is seen by sexual health charities as a high-risk period for both unwanted pregnancy and sexual infections.
One organisation offering abortion has said that demand for its services usually rises significantly in the first three months of the year.
Emergency contraception is effective for the first 72 hours after sex, but is more likely to work the sooner it is taken.
It causes side-effects such as nausea and diarrhoea in some women who take it.
It has been available for several years directly from pharmacies and walk-in-clinics, but BPAS fears that its £25 cost at pharmacies in some parts of the UK, combined with Christmas holiday closures, could discourage women from getting hold of it when needed.
Women will be sent the drug in the mail, with a supply of condoms, once their medical history and understanding of emergency contraception have been checked by the nurse on the telephone.
‘Risky’
Tracey Forsyth, one of the contraceptive specialist nurses working at BPAS, said: "We know that women often do not take the morning-after pill after unprotected sex. They may not think their risk of pregnancy is high, and the cost, inconvenience or embarrassment of obtaining it may put them off.
"Having it at home means you are much more likely to take it as soon as you need it.
"Sometimes women worry that requesting the pill in advance makes it look like you are planning on taking chances.
"In fact the opposite is true – making sure you have a back-up to help prevent an unwanted pregnancy is making sure nothing is left to chance."
The initiative was welcomed by sexual health charity FPA, which said it supported anything that might cut the number of unwanted pregnancies.
However, Life, a charity which opposes abortion, said there was no evidence that emergency contraception reduced unplanned pregnancy rates.
A spokesman said: "In fact, if a woman has the morning-after pill at home ‘just in case’ she may be more likely to engage in risky sexual behaviour than she would normally – particularly over the festive period with the associated increase in alcohol consumption."
