I'm flying from Porto (OPO) to London Stansted (STN) and then from STN to Warsaw Modlin (WMI), about two hours afterwards. Both flights are with Ryanair. I wanted to buy some Port wine at the duty free zone in OPO, to take as a gift, hoping it would be cheaper than paying for two checked bags. However, I'm not even sure if this is possible. Will I need to go through security again in STN? If so, is there a way to do this?
Note that this is specific to European airports as listed in the question. It is not applicable worldwide as pointed out by @jpatokal.
Yes, according to this European directive:
4.1.3 Screening of liquids, aerosols and gels (LAGs)
220.127.116.11 LAGs carried by passengers may be exempted from screening with LEDS equipment upon entry to the SRA in the following cases:
(a) if the LAG is in individual containers with a capacity not greater than 100 millilitres or equivalent in one transparent resealable plastic bag of a capacity not exceeding 1 litre, whereby the contents of the plastic bag fit comfortably and the bag is completely closed;
(b) if the LAG is sealed in a dedicated STEB upon purchase locally at the airport airside;
(c) if the LAG in a STEB originates from another EU airport or an aircraft of an EU carrier and is resealed in a dedicated STEB before leaving the security restricted area of the airport;
(d) if the LAG is screened locally with LEDS equipment airside and is then sealed in a dedicated STEB.
LAG: Liquids Aerosols and Gels
STEB: Secure Tamper-Evident Bag
Ensure that the duty free item is sealed in a STEB at OPO with the receipt visible inside the bag.