The Society for the Protection of Unborn Children (SPUC) is calling on the Government to delay implementation of the new Relationships and Sex Education (RSE) curriculum until September.
RSE became compulsory in schools across England and Wales last September but the Government has given until spring for delivery of the curriculum owing to the pandemic.
Schools are legally required to consult with parents before commencing RSE lessons but SPUC says these consultations have been thrown into jeopardy because of Covid-19.
The pandemic has caused chaos to schools, which are closed in the current national lockdown and are not likely to re-open until March 8 at the earliest.
SPUC has warned of the "dangers" of RSE and is calling on education secretary Gavin Williamson to delay delivery of the curriculum until parents have been properly consulted.
"Unfortunately, the extra burdens facing schools have only continued because of the ongoing pandemic," said Dr Tim Rogers, who heads up SPUC's Safe at Schools campaign.
"Consequently, many schools have still either not been able to hold the required parental consultations or the process of consultation has been severely disrupted or curtailed.
"This is no time to introducing new compulsory subjects to the curriculum, especially ones of such a controversial and sensitive nature, which require careful consultation and planning.
"It is all the more dangerous that parental consultations have been jeopardised, because parents no longer have the automatic right to withdraw their children from the new subjects."
SPUC is asking people to write to their MPs to petition Williamson on delaying RSE until September and reinstating the parental right to withdrawal.
"Parents should not have any form of Relationships and Sex Education foisted upon their children without being properly consulted," Dr Rogers added.