The EU plans to use its 60th anniversary celebrations in Rome to issue a warning that countries that leave the bloc will be “sidelined by global dynamics”, a leaked document reveals.

According to a draft of what is set to become the Rome declaration, the 27 states remaining after the UK leaves will say that unity is essential in the face of unprecedented challenges.

The EU does not plan to make a direct reference to the UK, but it will belittle the aspirations of those who believe there is a better future outside the union, according to the proposed wording.

“We are determined to make the EU stronger and more resilient; we need to show even greater unity and solidarity among EU member states. Unity is a necessity, not an option,” the draft declaration says. “Taken individually, we would be sidelined by global dynamics. Standing together is our best chance to influence them, and to defend our common interests and values.”

The Rome summit is to be held on 25 March. The European commission and the event’s Italian hosts have not invited Theresa May to the celebrations. In 2007, Tony Blair represented the UK at a grandiose 50th anniversary party for the EU, during which the member states issued a declaration calling for a “renewed common basis”.

According to the draft seen by the Guardian, the Rome declaration will begin by celebrating the formation of the union and its achievement in keeping the peace. “We, the representatives of 27 member states of the European Union, take pride in the achievements of the EU. The construction of European unity is a bold, far-sighted endeavour. Sixty years ago, recovering from the tragedy of two world wars, we decided to bond together and rebuild our continent from its ashes,” it says.

While stressing the importance of unity, the EU will say that member states should be able to integrate at different speeds. The German chancellor, Angela Merkel, has in recent months spoken of a two-speed Europe and the declaration is expected to emphasise the need for strengthened “common security and defence” and a “a Europe with a more integrated defence industry”.

It also appears that the EU will call on member states to “work together to promote the common good, on the understanding that some of us can move closer, further and faster in some areas, keeping the door open to those who want to join later, and preserving the integrity of the single market, the Schengen area, and the EU as a whole ... an undivided and indivisible union, which acts together whenever possible, at different paces and intensity whenever necessary”.

The document will be discussed by leaders on Friday during an informal summit for the EU27, which has been attached to the European council meeting staged in Brussels on Thursday.

May will attend the summit on Thursday but leave before the rest of the member states discuss their future together.