Equality groups and campaigners have slammed an open letter from Havering church leaders that argues “human stability” would be “confused and undermined” if plans to introduce same-sex marriage in Britain go ahead.

David Banting, vicar at St Peter’s Church in Harold Wood, is one of 10 ministers who have signed the controversial document calling on parliament to “think again” when the Marriage (Same Sex Couples) Bill comes back before it next week.

“We believe the average man and woman in the street do not want the law to go further than it has already done,” he told the Romford Recorder.

Rev Banting said he believed Hornchurch and Upminster Tory MP Dame Angela Watkinson had “rolled over” in voting in favour of the bill earlier this year.

The letter, sent to the Recorder this week, opines that allowing gay couples to get married represents an “ill-founded and unnecessary” change to marriage legislation.

It adds: “Redefinition would actually change marriage into something that is altogether less than marriage – in fact, not marriage at all. […] Human stability and flourishing would be confused and undermined.”

Rev Sharon Ferguson, chief exec of the Lesbian and Gay Christian Movement, called the letter “scaremongering”.

“How do two people celebrating their love for each other and being allowed to call it marriage undermine anybody else’s relationship?” she said.

The ministers’ letter says heterosexual marriage is “an inimitable partnership” and that children “need […] that unique union of male and female love”.

But Rev Ferguson said increasing numbers of children have been brought up in single-parent families since the Second World War.

Meanwhile, Dame Watkinson refuted the accusation that she had “rolled over”.

“I am under no pressure whatsoever on this,” she said. “It is a free vote. I am supporting it because I believe it is right.”

Father John-Francis Friendship, who was rector of the parish of St Andrew, Romford until 2011, and is openly gay, responded with a letter of his own.

“Whilst some church leaders may feel the ‘silent majority’ do not wish for marriage to be ‘redefined’ this assertion is not supported by any evidence,” it reads.

“A YouGov poll in March 2013 found only 32% opposed to same-sex marriage.

“My own limited experience indicates there is a significant percentage of Christians who support same-sex marriage.”

Romford MP Andrew Rosindell, who voted against the bill in February, said he believed the bill should be withdrawn and the subject of gay marriage consulted on more fully.

“Where would it end?” he said. “You could finish up at a stage where the monarchy in this country is in a same-sex marriage and that would have constitutional implications.”