Switzerland's central bank has announced it was introducing negative interest rates in an attempt to stop the Swiss franc from gaining further value.

The Swiss National Bank is imposing a rate of -0.25% on certain bank deposits, with the aim of pushing the target range of a benchmark interest rate into negative territory.

The rate on sight deposits, funds that can be accessed immediately, will come into force on 22 January and only apply to balances above a certain threshold.

The SNB said the aim was to take the three-month Libor rate, which Switzerland uses to determine interest rates on mortgages and savings accounts, into negative territory.

The target range for Libor - officially the Swiss franc's three-month London interbank offered rate - is now between -0.75% and 0.25%, down from between 0.0 and 0.25%.

Analysts have been expecting the bank to push rates into negative territory, which is designed to make it less attractive to hold Swiss franc investments.

The SNB reiterated its "utmost determination" to stop the Swiss currency gaining value and to keep to an exchange-rate floor of 1.20 francs to the euro, in an attempt to protect the country's vital export industry.

"Over the past few days, a number of factors have prompted increased demand for safe investments," it said.

"The introduction of negative interest rates makes it less attractive to hold Swiss franc investments, and thereby supports the minimum exchange rate.

"The SNB is prepared to purchase foreign currency in unlimited quantities and to take further measures, if required."