At their annual assembly in Geneva, the 192 nations in the World Health Organisation were expected to ratify a treaty hailed as a major breakthrough by the anti-smoking pressure groups.
Britain, which outlawed tobacco advertising in February, is believed to have pressed the US and Germany to sign up to the global accord.
After four years of negotiations, the treaty will commit its signatories to bans or strict controls over the advertising and marketing of tobacco, health warnings on cigarette packets and action to stamp out tobacco smuggling.
The last-minute decisions by the US and Germany to endorse the treaty were a surprise, but both nations left themselves some room for manoeuvre.
To have legal force in America, the pact will have to be signed by President George Bush and he has yet to decide whether to do so. Germany was given a special opt-out by European Union health ministers, including Alan Milburn, the health secretary, so that its objections did not scupper the international campaign.