The Prime Minister is being called upon to defend the right of Christians to express their faith in the workplace.

Advocacy group Christian Concern has today launched a campaign, Not Ashamed of the Cross, asking David Cameron to amend the Government's statement to the European Court of Human Rights (ECHR) on wearing a cross at work.

The Government stated that wearing a cross was not "a generally recognised form of practising the Christian faith".

The submission was made as the ECHR considers four cases brought by British Christians who were penalised for expressing their faith in the workplace.

They include nurse Shirley Chaplin, who was removed from ward duties after refusing to take off a cross necklace she had worn to work for thirty years.

She and three other Christians took their cases to court in the UK but lost. In its submission to the ECHR, the Government supported the original judgements, claiming that the religious freedom of the Christians had not been infringed.

The launch of Christian Concern's campaign comes days after the Prime Minister told church leaders at a Downing Street Easter reception that he supported the "fight back" by the church against rising secularism, and that Christian values were the values that "we need".

However, Christian Concern said the Prime Minister's public statements of support do not reflect the Government's position on paper.

In a letter to the Prime Minister, Ms Chaplin wrote: "[The Government's submission to the ECHR] claimed that, since I was free to practise my faith outside of work, and also that I was free to resign my job, my freedom of religion had been sufficiently guaranteed! Surely this is no freedom at all.

"In light of your views, reported this week, I urge you Prime Minister not to wait until the cases are heard by the court, but to act decisively now, ensuring that revised submissions are made expressing the Government's support for the vital principles of freedom of religion and conscience that are at stake."

Andrea Minichiello Williams, head of Christian Concern, urged the Prime Minister to set out his support for British Christians in clear policy terms.

She said: "The Prime Minister's words of support for Christians this week are empty and hollow unless he translates them meaningfully into actions which support and protect Christians who find themselves increasingly marginalised in the public sphere.

"Shirley Chaplin, Gary McFarlane and the others who now face the European Court of Human Rights in their pursuit of justice, are calling on the Prime Minister to act on his stated convictions and on what he knows to be right.

"If the Prime Minister is to be trusted he needs to carry through on what he says he believes; otherwise he will be a man who disappoints, a man who appears like a chameleon, changing his tune according to whoever he is talking to.

"This Easter, if the Prime Minister wants people of faith to take him seriously, then we call on him to act according to his words otherwise they will seem to be empty rhetoric."

The Prime Minister's aides told leaders at the Downing Street reception that the Government will bring in legislation to support the four Christians if the ECHR rules against them.

Ms Minichiello Williams added: "Why make the process so difficult? Our own courts have already gone against Christians, and he and his Government know it. Why not simply change the Government's official position in Europe to help secure freedom for Christians now?"