The UK’s opt-out from the single currency appears in Protocol 15 to the Treaties. Point 1 reads:

1. Unless the United Kingdom notifies the Council that it intends to adopt the euro, it shall be under no obligation to do so.

This protocol does not expire at some point, as is sometimes suggested. Neither are ‘all Member States obliged to join the euro by 2020’. So the opt-out is valid for an unlimited period.The protocol goes on to disapply the various EU law rules relating to the single currency. This has a number of implications. Due to the single currency opt-out the UK cannot be subject to austerity measures imposed by the ‘Troika’ that oversees bail-outs to Eurozone countries, since this only applies to states which adopt the single currency. Austerity policy in the UK is solely a decision made by our own government.

Source: EU Law Analysis: EU Referendum Briefing 1: Can the UK control the EU’s future if it stays a member?