Home Secretary Theresa May has admitted "international relations" were a factor in the Government's decision not to hold a public inquiry into the death of poisoned spy Alexander Litvinenko.

Coroner Sir Robert Owen had requested the Government order the inquiry because he could not consider vital secret evidence as part of a normal inquest.

This was backed by Mr Litvinenko's widow Marina, who said she and her legal team were "shocked" by the Government's refusal.

Yesterday, a letter from Mrs May to Sir Robert explaining the reasons for the decision was published.

It said: "It is true international relations have been a factor in the Government's decision-making.

"An inquest managed and run by an independent coroner is more readily explainable to some of our foreign partners, and the integrity of the process more readily grasped, than an inquiry, established by the Government, under a chairman appointed by the Government.

"However this has not been a decisive factor."

Mrs May said the Government is anxious that Mr Litvinenko's death is properly investigated, and accepts there are "important factors" in favour of establishing an inquiry.

She wrote: "The Government is anxious that as much as possible of the investigation is conducted in public."

Mr Litvinenko, 43, was poisoned with radioactive polonium-210 in 2006.

Since his death, his widow and son Anatoly have battled to discover the truth.