By Christopher Hope, Senior Political Correspondent

Last month the Prime Minister pledged “no-one should be in any doubt: prisoners are not getting the vote under this Government”, despite a ruling seven years ago by the European Court of Human Rights that they should be allowed a say at elections.

Justice Secretary Chris Grayling is publishing legislation on Thursday setting out three options, including an outright ban. The other options will be for prisoners to get the vote if they are serving jail terms of fewer than six months or fewer than four years.

However, Lord Lester of Herne Hill QC, who sits the Ministry of Justice’s Commissin which is drawing up a new British Bill of Rights, said offering a ban was merely political posturing, and it was inevitable prisoners would get the right to vote.

Asked if the Government would have to the vote to prisoners in some form, he told The Daily Telegraph: “Of course – either that or we are in the same position as in Greece under junta. Greece had to leave the Council of Europe.

“We are talking about a convention that was invented by Winston Churchill, Harold Macmillan, Anthony Eden – Conservatives who believed in the European rule of law. We have always abided by judgements of the Strasbourg court.”

Lord Lester, who sits on the Liberal Democrat benches in the House of Lords, said Mr Grayling was only offering the option “to placate the right wing of the Conservative party” and not because it had real chance of sticking.

He said: “What it actually does is that it encourages more irresponsible behaviour in the guise of democracy. It is a profoundly important matter.”

Lord Lester continued: “Think what would happen in Russia – if they were to adopt a tactic of saying to the Duma [Russian Parliament] that they did not like a judgement.

“That would be irrelevant to the real question which is: has the Russian Federation complied with the judgement?”

The legislation is being published on Thursday, a day before the deadline to comply with the ruling set by officials at the ECHR in Strasbourg, which has the power to issue fines for any breach.

Last February, the Commons called by an overwhelming margin of 234 to 22 for the blanket ban to be maintained and David Cameron has flatly ruled-out defying parliamentary opinion.

Lord Lester said the continued uncertainty over the affair was a “running sore” which “gravely undermines our reputation” on the international stage.

He added: I am told that in Beijing two weeks ago a British delegation was there to discuss the rule of law and some of the senior Chinese officials said ‘oh then will you please tell us why Britain is flouting the judgement of the Strasbourg court’. If that has reached Beijing then it stains our fine international reputation of the rule of law.”

The Prime Minister told MPs last month that he was happy to introduce legislation to “help put the legal position beyond doubt. But no-one should be in any doubt: prisoners are not getting the vote under this Government.”

The European Court of Human Rights has said countries can decide which prisoners should be denied the right to vote from jail, but cannot impose a blanket exclusion.

Shadow home secretary Yvette Cooper said a vote on legislation would strengthen the UK’s argument that it should retain its ban – which is backed by Labour.

A Ministry of Justice spokesman said: “The Government is considering how best to proceed following the judgment of the Grand Chamber of the European Court of Human Rights. An announcement will be made to Parliament shortly.”