The phrase "a formal pact" is correct and usable in written English.
You can use it when referring to a serious agreement or treaty between parties, often in a legal or diplomatic context.
Example: "The two countries signed a formal pact to enhance their cooperation on trade and security issues."
Alternatives: "an official agreement" or "a binding contract."
Last Thursday, Miliband used the BBC's challengers' debate to dramatise his refusal to consider a formal pact with the SNP.
The tax break was passed in 2006, but the union never hammered out a formal pact with the club owners.
First, there are no mechanisms in any of the formal party structures to organise a formal pact.
Some Conservative lawmakers have called for a formal pact, warning that the U.K. Independence Party could draw votes away from them, possibly depriving them of an election victory.
But after the Poitevins were expelled, Edward fell under the influence of Simon de Montfort, his uncle by marriage, with whom he made a formal pact.
A good way out of this mess would be for Washington to take up the proposal made Friday by Germany and France to negotiate a formal pact that would set mutually acceptable surveillance guidelines.
David Cameron is right to reject any formal pact with Ukip.
But the Barclays and R.B.S. offers represent the most significant progress yet - and the only formal pact with a coalition of customers, rather than ad hoc agreements that other banks and investors arranged with individual clients.
Writing in The Times, the UKIP leader says that he is despised by David Cameron, and a formal electoral pact with the Conservatives would be nonsensical.
A few months later they joined forces with the rising Labour star Lisa Nandy to produce this book, arguing not for a merger of their three parties, or even a formal electoral pact, but more cautiously for making common cause wherever possible.
Martin Boon of ICM said: "There are signs here of voters being anxious about a minority government surviving by scrambling around for minor parties' votes, preferring the stability of a formal coalition pact, and as the leader of a coalition that has lasted five years that would seem like good news for David Cameron".
