Update 2018-07-05: According to current media reports on Wednesday evening (04 July, local time), the couple taken critically ill were poisoned by the nerve agent Novichok.
2018-07-04: According to a police report from late Tuesday evening (03 July, local time), two people showing symptoms of poisoning were hospitalised in Amesbury, around ten kilometres north of Salisbury in Southern England, on Saturday (30 June). The toxic substance both people had been exposed to was not identified yet. It was initially thought, that the symptoms of poisoning result from a drug overdose. A former Russian intelligence official had been poisoned in Salisbury in March 2018.
The police declared a major incident. However, it is believed, that there is no significant health risk to the public. Travellers should follow instructions from authorities.