A former senior diplomat has warned that Britain has gone a "step too far" by lifting a block on the death penalty of two Islamist terrorists likely to be extradited to America.

Recently retired George Fergusson joined a chorus of objections to the decision, made quietly by Home Secretary Sajid Javid this month after years of the UK never co-operating with overseas capital cases.

Mr Ferguson, a Scot who worked in the cabinet office until he stepped down last year, fears the move could make it harder for Britain to hold up the human rights in the future.

His views echo those of opposition leaders and backbenchers who have criticised Mr Javid for signalling he would co-operate with a US prosecution of Alexanda Kotey and El Shafee Elsheikh, two British Jihadis captured by Kurds in Syria.

The men, along with two others, had been nicknamed The Beatles, and have been accused of taking part in the 2014 murder of American journalist James Foley. That allegation paves the way for Kotey and Elsheikh, who have been stripped of their UK citizenship, to face trial in the United States.

British authorities are believed to have information to support such a prosecution. Mr Javid has not sought the normal reassurances that such material will not contribute to a capital case.

Mr Fergusson, a former Governor of Bermuda and High Commissioner in New Zealand, described this move as "curiously discreet".

He said: "If there are people whom we believe to have done appalling things, including to British citizens, and there is a prospect of bringing them to justice, we should try to do that.

"If, as seems to be so, the legal advice is that they probably wouldn’t be convicted in British courts, it is legitimate to look at other respectable options.

"A balance has to be struck. But undermining proper criminal law systems is a step too far.

"That is one concern about Sajid Javid’s decision. Another is that a major change of policy has been taken without public notification."

Mr Fergusson said there were other "unsettling features" in the decision.

He said: "One is that, having found in the US Federal Courts a respectable legal system where a successful prosecution might be possible, it seems that we can only persuade the US authorities to take on the task if there is a prospect of killing the defendants, if convicted. I hope that isn’t true. But it is hard to see any other explanation for the UK Government’s waiver.

"And, secondly, the suggestion that this is a one-off, and our underlying policy is unchanged, is not credible."

Mr Fergusson cited his own experience in dealing with the shoebomber, Richard Reid, the British terrorist who tried to blow up a transatlantic jet shortly after 9/11.

He said: "When Reid made his unwelcome appearance in Boston on an American Airlines flight which he had tried to blow up, I was the British Consul General.

"It was clear, at least to me, both that he had tried to do something truly awful – and that our right and obligation to visit British nationals in custody applies as much in hard cases as easy ones. The US authorities initially refused access. Our Embassy in Washington discouraged me from pushing further: we needed to show our solidarity on terrorism after 9/11, only three months earlier. I agreed on solidarity but reckoned that if we conceded once that the right of consular access was at the host country’s discretion – even America’s – it would logically always be discretionary rather than a right.

"That is the danger over the death penalty waiver. If we have given in on this case, however tempting, how do we justify not bending the rules in future?"