A letter signed by 22 current and former chairmen of local groups urges the prime minister to reconsider or at least delay the plans, which face a crucial vote this week.

With 303 Conservative MPs and more than twice that many parliamentary constituencies in the UK, the signatories officially represent a small proportion of the party grassroots party. But their intervention will be taken seriously with growing signs of unhappiness among Tory MPs and their supporters. Even MPs who support equal marriage laws report they are inundated with emails and letters objecting to the move and only a small handful are in support.

Sunday newspapers report that up to four cabinet ministers and potentially more than half of Conservative MPs are considering voting against the bill on Tuesday – which despite being a free vote would be an embarrassment to the Tory leadership and especially for David Cameron, who has been so closely associated with the proposal.