A Scottish Catholic bishop has condemned David Cameron for being “devoid of moral competence” after the PM stated that churches should be allowed to provide same-sex marriages.

The Rt Rev Joseph Devine, Bishop of Motherwell, believes the prime minister is now “out of his depth” and can no longer be trusted by Christians.

He has also accused the PM of undermining family life for allegedly refusing to support moves by Christians at the European Court of Human Rights (ECHR) to demand their right to wear the cross at work.

In a letter, the bishop compared David Cameron to Nero, the brutal Roman emperor who persecuted Christians.

He wrote: “You vacillate, ambivalent about the role you wish to perform – the disciple of David or Nero”.

“With such a contradiction between your statements and actions, on what basis can you expect anyone – Christians in particular – to trust or respect you?”

The Telegraph reports that Bishop Devine has also criticised Mr Cameron for commenting on last month’s decision of the Church of England General Synod to vote against ordaining female bishops.

In response, a Downing Street spokesman declined to comment on the letter but said the debate on equal marriage was “about how rather than if” it would go ahead.

Mr Cameron has said he is resisting ECHR action on the cross as he wants such issues dealt with in British courts.

In September, Bishop Devine criticised the Green Party for taking disciplinary action against a Christian councillor in Brighton who voted against the party over the issue of equal marriage.