Britain's Prime Minister, Theresa May, leaves 10 Downing Street, in central London, Britain March 21, 2018. REUTERS/Toby Melville

BRUSSELS (Reuters) - EU leaders will issue a warning to Britain on Friday that it will only finally secure a Brexit transition period if it stands by its commitments on the Irish border and Gibraltar, a draft statement showed on Wednesday.

The 27 leaders, who will meet Prime Minister Theresa May on Thursday, will endorse an interim deal struck by EU negotiators on Monday to grant London a 21-month, status quo transition period to help businesses adjust to Brexit, due in March, 2019.

A written declaration reflecting that was held up for redrafting after Spain sought a reiteration of its interests being protected over Gibraltar, the British territory on its south coast, which will also be leaving the European Union.

Summit chair Donald Tusk said on Wednesday that Spain had agreed to the new wording.

Ireland has also been anxious to ensure Britain makes good on promises to solve difficulties on their mutual border.

The text reminds London that the agreement on the transition will only become legally binding once the whole treaty on Britain’s exit from the bloc is agreed and ratified, probably not until early next year.

“The European Council welcomes the agreement reached by the negotiators on parts of the legal text of the Withdrawal Agreement covering citizens’ rights, the financial settlement, a number of other withdrawal issues and the transition,” it read.

“The European Council recalls that other issues still require agreement and negotiations can only progress as long as all commitments undertaken so far are respected in full, and welcomes in this respect Prime Minister May’s written assurances notably regarding Ireland/Northern Ireland.”

“The European Council calls for intensified efforts on the remaining withdrawal issues as well as issues related to the territorial application of the Withdrawal Agreement, notably as regards Gibraltar, and reiterates that nothing is agreed until everything is agreed.”