"You should be aware that there is a legal basis in law in the UK (the Diplomatic and Consular Premises Act 1987) that would permit us to take steps to arrest Mr Assange within the current premises of the embassy. We sincerely hope that such a point is not reached, but if you cannot resolve the presence of Mr Assange on your premises, that route is open to us."
The only possible interpretation of the last sentence is that if the government of Ecuador does not expel Mr Assange from the Ecuadorean embassy in London, the British government has the option "to arrest Mr Assange in the current premises of the embassy", and will if necessary do so. It is a clear threat to the integrity of the embassy, and no amount of back-peddling by Mr Hague by describing it as only an explanation of UK law can now unfortunately disguise the fact that this aide memoire was written to the Ecuadorean government with the text referred to, and so this threat, contained in it.
So what is the legal position? Article 22 of the Vienna Convention 1961 codifies the long-standing principle of the immunity of diplomatic premises ("the mission") in unambiguous language, in the following words:
"1. The premises of the mission shall be inviolable. The agents of the receiving State may not enter them, except with the consent of the head of the mission.
3. The premises of the mission, their furnishings and other property thereon and the means of transport of the mission shall be immune from search, requisition, attachment or execution".
So entering an accredited diplomatic mission without the consent of the ambassador appointed there is quite simply a breach of public international law.
As a side issue it is worth noting that foreign diplomats are not immune from the laws of the host country as is sometimes suggested, and in this case Ecuadorean diplomats are quite probably acting unlawfully according to the law of England and Wales in aiding Mr Assange's breach of his bail conditions, albeit they have diplomatic immunity from prosecution for so doing. However, the fact that there very probably has been and continues to be a breach of English and Welsh law with respect the aiding of Mr Assange's escape from extradition in breach of bail conditions does not affect the inviolability of the premises of Ecuador's diplomatic mission.
And is the letter an accurate description of UK law anyway? It is not. Section 1 of the Diplomatic and Consular Premises Act 1987, passed in part in response to the events at the Libyan embassy in St James's Square culminating in the death of PC Yvonne Flether, is in these terms:
"1 (1) Subject to subsection (2) below, where a State desires that land shall be diplomatic or consular premises, it shall apply to the Secretary of State for his consent to the land being such premises.
(2) A State need not make such an application in relation to land if the Secretary of State accepted it as diplomatic or consular premises immediately before the coming into force of this section.
(3) In no case is land to be regarded as a State’s diplomatic or consular premises for the purposes of any enactment or rule of law unless it has been so accepted or the Secretary of State has given that State consent under this section in relation to it; and if—
(a) a State ceases to use land for the purposes of its mission or exclusively for the purposes of a consular post; or
(b) the Secretary of State withdraws his acceptance or consent in relation to land,
it thereupon ceases to be diplomatic or consular premises for the purposes of all enactments and rules of law.
(4) The Secretary of State shall only give or withdraw consent or withdraw acceptance if he is satisfied that to do so is permissible under international law.
(5) In determining whether to do so he shall have regard to all material considerations, and in particular, but without prejudice to the generality of this subsection—
(a) to the safety of the public;
(b) to national security; and
(c) to town and country planning.
(6) If a State intends to cease using land as premises of its mission or as consular premises, it shall give the Secretary of State notice of that intention, specifying the date on which it intends to cease so using them.
(7) In any proceedings a certificate issued by or under the authority of the Secretary of State stating any fact relevant to the question whether or not land was at any time diplomatic or consular premises shall be conclusive of that fact."
These provisions would indirectly allow the Secretary to enter the premises of a diplomatic mission, but only after he has withdrawn his acceptance and consent to the use of the embassy premises as a diplomatic mission in conformity with section 1. To obtain Mr Assange he would have to expel the entire diplomatic mission from the UK, including the embassador, and sever diplomatic relations with Ecuador. In doing so, under section 1 Mr Hague would still need first to act in accordance with international law, and secondly to meet the requirement for him to "have regard to all material considerations". A declaration of war would certainly justify expelling the entire mission in international law, but the extent to which lesser acts might do so is not spelt out in the Convention.
In any event closing the entire mission would almost certainly fail to withstand scrutiny in the domestic courts, were the diplomatic mission to bring such proceedings. The requirement to have "regard to all material considerations" is a term of art which requires rationality on the part of the Foreign Secretary, and so enables the administrative courts to consider whether withdrawing diplomatic consent to the siting of the embassy was within the range of the things that a rational Secretary of State could do, after having considered all material factors. Given that harbouring a fugitive who has been arrested (but not charged) by the relevant Swedish authorities for amongst other things rape, and who could (and would) be arrested as soon as he steps outside the embassy, does not in practice bear on safety, national security or the amenity of the area neighbouring the embassy even if he did commit the offences for which he has been arrested (which he contests), then withdrawal of consent would be likely to fail in UK law if challenged. It is also to be noted for the purposes of section 1(3)(a) that the embassy in Knightsbridge is a full mission and not a "consular post", so any argument that harbouring a fugitive from justice does not comprise use "exclusively" as a consular post is irrelevant.
Declaring a state of war would be even more absurd, albeit free from the intervention of the domestic courts even if not from domestic and international ridicule.
Possibly Mr Hague's explanation that this aide-memoire is not the threat which its plain text indicates it is, is to be construed as an indication that the UK government propose, if necessary in order to secure Mr Assange's arrest, to sever diplomatic relations with Ecuador and/or declare war. If so, that is such a gross overreaction and act of diplomatic stupidity as scarely to improve his position. In either case, he shows himself as lacking both judgement and a grip on his own department.