Marry in Haste — Jane Aiken Hodge

Camilla Forest, a lovely young refugee from France, had married the proud and sardonic Lord Leominster. There was only one problem: she was falling in love with her husband. And they had agreed that theirs was to be a marriage in name only.

Suddenly, Leominster was sent to Lisbon on a diplomatic mission. Or so Camilla thought, until she discovered that he was really a spy. Now both their lives depended on the success of a masquerade that she alone could play ...
------------------------------------
UK Hodder Edition:

Lord Leominster married Camilla for convenience only - yet she found herself falling ever more deeply in love with him!

At a time when England and Napoleonic France were about to lock horns, Leominster embarks on a secret diplomatic mission to Portugal, where the French army is about to strike.

Camilla and her flighty sister-in-law accompany him and cruel circumstance plunges them all into disaster.

Separated from her husband, Camilla spends the winter in hiding from the French and more intimate enemies. But after a harrowing escape, their reunion is fraught with surprise - for Leominster, it was the most incredible surprise of all.