I have been in the UK holidaying for a while and both my Siam Commercial Bank and Bangkok Bank Cards have worked perfectly with UK ATM's. As of this weekend BOTH ceased to work (I've checked at several different ATM's including ATM's which up to now have worked fine). Anybody know why that might be?. Hope somebody can advice.

Maybe the banks think your card is stolen and stopped services?

Two cards from different banks both at the same time - seems that blockage of those specific cards unlikely.

Could be an internet issue from the ATM network back to the Thai banks, or coincidentally they are both down for system maintenance, or share the same systems.

Calling the bank would probably be the wisest move and if they worked before but continue to not now then it is almost surely due to security .. which a phone should be able to fix. It is ALWAYS advisable to let your bank know before you intent to use your ATM card overseas.

After several attempts to access your accounts (3) the accounts may lock and your password no longer valid. If that is the case then the bank may only offer to send you a replacement password and/or card only to your registered account address. If that is so you are screwed if your address is in Thailand. I would consider going to the Bangkok Bank in London and asking for assistance.