What is the significance of Margaret Thatcher's entry into the Exchange Rate Mechanism in 1990?

Economic significance: The decision to enter ERM in 1990 was economically detrimental as this occured at a time when the £ was valued too highly compared to the Deutschmark, meaning the exchange rate could not be locked into place to hold down inflation. The result was the 'Black Wednesday' monetary crisis of 1992 which cost the government around £3.5 billion and a significant depletion of foreign exchange reserves.

Political significance: The decision highlighted the lack of cohesion in Thatcher's cabinet as she was advised by Chancellor Lawson to enter the ERM in 1985 however refused to do so. The crisis of 1992 worked to galvanise further Eurosceptism in the Conservative party, impeding John Major's plans to integrate Britain further with the EU, and arguably was one of the main events that tipped the scales for the 'leave' side in 2016.