By Julia Payne LONDON (Reuters) – Russia’s Gazprom has told customers in Europe it cannot guarantee gas supplies because of ‘extraordinary’ circumstances, according to a letter seen by Reuters, upping the ante in an economic tit-for-tat with the West over Moscow’s invasion of Ukraine. Dated July 14, the letter from the Russian state gas monopoly, said it was declaring force majeure on supplies, starting from June 14. Known as an ‘act of God’ clause, force majeure is standard in business contracts and spells out extreme circumstances that excuse a party from their legal obligations. Gazprom’s h…