Philip Hammond has sided with four other European treasury ministers to warn Donald Trump that he could start a trade war with a series of proposed tax reforms.

In a highly unusual move, Britain, France, Germany, Italy and Spain have joined forces in writing a letter to the US Treasury expressing "significant concerns" about the tax changes "having a major distortive impact on international trade".

The ministers said the proposed changes could "contravene" World Trade Organisation (WTO) rules and America's own tax guidelines. President Trump wants to bring in a series of measures to increase the tax burden on foreign companies operating in the US.

Mr Hammond's decision to sign the letter amounts to a political gamble because it risks angering Mr Trump at a time when Britain is trying to strengthen its trade links with the US.

The Europhile Chancellor has also yoked himself to four other EU countries at a particularly sensitive point in the Brexit negotiations.