Ireland's ambassador to the UK warned that Brexit will make it more difficult to achieve a co-ordinated international response to incidents like Salisbury in the future.
Adrian O'Neill told BBC Radio 4's The Westminster Hour: "The UK leaving the European Union doesn't make the task of achieving a coherent solidarity in response to an event like Salisbury any easier.
"It does make me wonder in the future, once the UK has left the European union, that task of ensuring that the necessary solidarity is there among like-minded countries will be more challenging with the UK outside the European Union than within.
"I suppose even just in terms of the process and procedure that are required to arrive at common positions and statements and so on, are transacted more easily when you are part of a collective than if you have to do them as a series of bilateral transactions.
The European Union has issued a statement voicing "unqualified solidarity" with the UK over the Salisbury attack.
The statement, agreed by foreign ministers from the 28 member states meeting in Brussels, called on Russia to "address urgently the questions raised by the UK and the international community and to provide immediate, full and complete disclosure of its Novichok programme to the Organisation for the Prohibition of Chemical Weapons".
"The European Union strongly condemns the attack that took place against Sergei and Yulia Skripal in Salisbury, UK on 4 March 2018, that also left a police officer seriously ill.
"The lives of many citizens were threatened by this reckless and illegal act. The European Union takes extremely seriously the UK Government's assessment that it is highly likely that the Russian Federation is responsible.
"The European Union is shocked at the offensive use of any military-grade nerve agent, of a type developed by Russia, for the first time on European soil in over 70 years. The use of chemical weapons by anyone under any circumstances is completely unacceptable and constitutes a security threat to us all.
"Any such use is a clear violation of the Chemical Weapons Convention, a breach of international law and undermines the rules-based international order.
"The EU welcomes the commitment of the UK to work closely with the Organisation for the Prohibition of Chemical Weapons (OPCW) in supporting the investigation into the attack.
"The Union calls on Russia to address urgently the questions raised by the UK and the international community and to provide immediate, full and complete disclosure of its Novichok programme to the OPCW.
"The European Union expresses its unqualified solidarity with the UK and its support, including for the UK's efforts to bring those responsible for this crime to justice.
"The EU will remain closely focussed on this issue and its implications."
Arriving at a meeting with EU counterparts in Brussels, Britain's Foreign Secretary Boris Johnson said: "Today the technical experts from the Organisation for the Prohibition of Chemical Weapons are arriving in the UK to take the samples from Salisbury, and meantime the Russian denials grow increasingly absurd.
"At one time they say that they never made Novichok, and at another time they say they did make Novichok, but all the stocks have been destroyed ... but some of them have mysteriously escaped to Sweden, or the Czech Republic, or Slovakia, or the United States, or even ... the United Kingdom.
Mr Johnson added: "What really strikes me, talking to European friends and partners today, is that, 12 years after the assassination of Alexander Litvinenko in London, they are not fooling anybody anymore.
"There is scarcely a country round the table here in Brussels that has not been affected in recent years by some kind of malign or disruptive Russian behaviour. And that is why, I think, the strength and the resolve of our European friends is so striking today."
The Foreign Secretary said: "I have been very heartened already by the strength of the support that the UK is getting in respect of the incident in Salisbury.
"And, I must say, I think that is partly because they can see that Britain is acting in punctilious accordance with our obligations under the treaty on chemical weapons.
"And I would contrast that with how the Russians are behaving."