German Chancellor Angela Merkel sought on Friday to assuage concerns in the U.K. about the Brexit deal, stressing that a controversial “backstop” arrangement is intended as an insurance policy, and would be temporary.

Speaking to reporters in Brussels, at the end of a two-day EU summit, Merkel reaffirmed the common position of the EU’s 27 member states that the U.K. withdrawal deal, reached with British Prime Minister Theresa May last month, is not open to renegotiation.

“The withdrawal agreement has been negotiated and will not be changed. It is important for us that we have a relationship between the EU and Great Britain in which each partner can develop freely,” she said.

Merkel underlined that the conclusions of Thursday's EU meeting contain assurances for the U.K.’s future relations with the bloc, amid concerns that the backstop arrangement could force London to follow EU rules for an indefinite time.

“We need this backstop only as an insurance policy, if we cannot manage in a timely way to conclude the transition period, conclude a final agreement on our future relationship,” she said.

Merkel argued the arrangement is important to prevent a hard border between Northern Ireland and the Republic of Ireland, and ensure the integrity of the single market.

She also underlined that all of the 27 member states want to establish a close partnership with the U.K. in the future, and are ready to work speedily to negotiate a future relationship after the U.K.’s approval of the withdrawal agreement.

The Brexit deal foresees the U.K.’s exit from the EU on March 29, and a transition period through the end of 2020, during which Brussels and London are expected to negotiate and conclude a new deal for their future relationship.

If no deal can be achieved, a “backstop” will automatically come into effect, to ensure no return to physical borders between Northern Ireland and the Republic of Ireland.