Theresa May has been accused of failing as prime minister for leaving key ministerial posts vacant, including one for so long that the MP who quit has offered to come back.

More than a week after a mini-cabinet reshuffle which saw Rory Stewart promoted to cabinet, his his old prisons minister role remains unfilled despite record levels of violence and self-harm in English jails.

And more than a month after respected former Middle East minister Alistair Burt quit over Brexit, his post remains unfilled.

Tensions are running high in the region, with Iran suspending its commitments under the 2015 nuclear deal after it was abandoned by Donald Trump’s US. As a result, Burt has made a serious and unprecedented offer to return to the government.

He told HuffPost UK: “The FCO is doing fine, as is my colleague Mark Field handling the Middle East and North Africa during a very busy period as its interim minister.

“In these slightly unusual times, being fully honest, I would love to return, and thank a number of people for their kind remarks on the subject.

“But I fully appreciate that having left the government, I’m not in a position to do anything but accept others decisions about government positions.”

Meanwhile the Liberal Democrats said May was presiding over an irresponsible government paralysed by Brexit, insisting the crisis in prisons demands urgent action.

The situation is so bad that Stewart last summer promised to quit if he had not managed to bring down drug and violence levels in 10 target English jails within a year.

But last week he was promoted to international development secretary to replace Penny Mordaunt, who in turn replaced sacked defence secretary Gavin Williamson, and the prisons minister role remains unfilled.

Lib Dem justice spokesperson Wera Hobhouse said: “Our prisons are in crisis, and Theresa May clearly doesn’t care. She is failing the most basic test of leadership by leaving the role of prisons minister empty.

“This crisis demands urgent action, not the total abdication of responsibility we’re seeing from this Tory government.

“But this not the only unfilled post, the government still haven’t replaced Alistair Burt who quit his post in the Foreign and Commonwealth Office over Brexit six weeks ago.

“The paralysis we are witnessing is a result of the Brexit filled abyss the government have dragged us all into.”

Labour party chairman Ian Lavery added: “The Tories are running a zombie government – posts have been left vacant for months and hundreds of urgent pieces of government business are lying on the cutting room floor.

“With 14m people living in poverty, rising homelessness and life expectancy stalling, the country is crying out for action. Sadly the Tories can only offer failure and disruption.”

Responding to questions on when the vacancies would be filled, May’s official spokesman told reporters: “We will announce those appointments in due course.”