British Foreign Secretary Philip Hammond speaks during a joint press conference | Debbie Hill/ EPA Brexit deal could take six years, says UK foreign minister Britain will also face legal obstacles to trade deals with non-EU countries, Hammond told MPs.

British Foreign Secretary Philip Hammond has stressed the difficulties of leaving the European Union, saying that a Brexit agreement could take up to six years and that the country's ability to negotiate trade agreements would be limited.

"The concern is this ... if a future treaty between the U.K. and the EU-27 is deemed to be a mixed competence, it will have to be ratified by 27 national parliaments. I think I am right in saying the shortest time in which that has been done in any EU treaty is just under four years, and that is after taking into account the time it has taken to negotiate," Hammond told parliamentarians on Tuesday, according to the Guardian.

The agreement to settle relations between the U.K. and the remaining 27 member countries after Brexit will not only concern trade -- which is an EU-only competence -- but will also cover other aspects of future EU-U.K. relations, such as security and foreign policy. It will therefore need to be ratified by all 27 parliaments.

Hammond also noted that Britain would face legal difficulties in negotiating trade agreements with non-EU countries. It has begun "preliminary" talks with India and has also announced that it will sound out post-Brexit arrangements with the U.S., China, South Korea and Japan.

"Until we have served an Article 50 notice, we remain a full, participating member of the EU and our ability to negotiate new trade agreements is restricted by the continued application of EU law until we have negotiated our exit from the EU," Hammond told British MPs. "We have to tread a careful path having any preliminary negotiations but remain on the right side of our international obligations," Hammond said.