Europe's most senior judge has warned that the government is in danger of undermining human rights legislation and thrown back accusations that the Strasbourg court is responsible for lengthy delays in extradition cases.

In an interview with the Guardian before stepping down as president of the European court of human rights (ECHR) at the end of the month, Sir Nicolas Bratza also says that the government's reluctance to grant prisoners voting rights is "damaging".

His comments are a robust response to criticism directed at Strasbourg and the ECHR by the home secretary, Theresa May, and the lord chief justice, Lord Judge.

In the wake of protracted appeals involving the terror suspects Abu Hamza and Abu Qatada, as well as the alleged computer hacker Gary McKinnon, concern has mounted about cases lasting as long as eight or 10 years.

In the Commons last week, May said: "We will work closely with the European court of human rights on a programme to reduce the wholly unacceptable delays that have occurred there."

Last month, Judge said legal holdups were "a source of real fury to me" and in reference to Hamza added: "I do not think … you will discover that the delays in that particular case, or in many like it, are actually to be levelled at the doors of the courts here."

Speaking in Strasbourg, Bratza said: "I have read what the lord chief justice has to say about it, where he exonerates his own courts and talks about 'the courts here', and the implication is that perhaps the Strasbourg court is to blame. I don't think that's fair if that's the implication.

"These were cases of considerable complexity. There was great co-operation between the parties. Never once did the UK complain that we were dragging our feet or taking too long. On the contrary, the UK, because of the need to find extensive evidence about supermax prisons, asked for a considerable extension of time to submit information.

"We acceded to that because it was important, but to blame the court for granting an extension that's been requested by the government … would be unfair. I don't accept for one moment that we were guilty of the substantial delay with which we seem to have been saddled.

"The returning of people to another country whether by deportation or extradition is very difficult."

The case of Abu Qatada, which is before the UK's special immigration appeals commission (Siac), might return to Strasbourg, he said.

The ECHR has accelerated its processing of cases, Bratza said, significantly reducing the notorious backlog. A victim of its own success, the court at one stage had 161,000 cases waiting to be dealt with. "That figure is declining now and we are making great progress. Now it's down to 138,000."

By using a single judge system and filtering out inadmissible cases "this year we have already achieved something like 70,000 cases. That's a phenomenal number of cases resolved. We were always accused of inefficiency. I think the figures disprove that." It is estimated that the backlog will be cleared completely by 2015.

Bratza also questioned why the coalition had launched a bill of rights commission to review the UK's obligations under the European convention on human rights. The commission is due to report by the end of the year. Eurosceptics hope it will dilute the Human Rights Act, while civil liberties groups are satisfied with the status quo.

"My problem is understanding why it's desirable to introduce a new bill of rights into law," said Bratza. "The [existing] Human Rights Act is an extremely skilful piece of legislation which does strike exactly the right balance.

"The [UK] courts faced with it are able to draw upon 50 or 60 years of established jurisprudence [at Strasbourg]. If you start introducing a new bill of rights and possibly new definitions and new provisions which have slightly changed the rights that are already covered by the convention, there's a risk of incoherence creeping in – particularly if it's going to stand side by side with the convention, which will still bind the UK nationally.

"Until [we] see what [a new bill] adds or takes away, it's difficult to form an assessment but I don't see the need. It's said it would make it more British and will build in the right to a jury trial … Do you need to build it in as a separate right? I'm doubtful."

Bratza also warned that the UK's reluctance to give prisoners voting rights – in defiance of repeated rulings by the ECHR – risked damaging the convention system. The final deadline for the UK to comply with the Strasbourg ruling is next month. In the meantime Strasbourg has accumulated several thousand related claims on voting rights from prisoners.

"The fact that there's been non-compliance with prisoners' rights has had an effect," Bratza said.

"For a country as important as the UK to be seen not to be implementing the judgment has had an impact. It's not led to any greater determination of other states to be defiant, but I do think that it's seen as damaging that a country as important as the UK has not complied with a court judgment yet."